Re: [PATCH v7 3/4] fs/proc/task_mmu: Implement IOCTL to get and/or the clear info about PTEs

[Muhammad Usama Anjum]

On 1/19/23 3:28 AM, Peter Xu wrote:
> On Mon, Jan 09, 2023 at 11:45:18AM +0500, Muhammad Usama Anjum wrote:
>> This IOCTL, PAGEMAP_SCAN on pagemap file can be used to get and/or clear
>> the info about page table entries. The following operations are supported
>> in this ioctl:
>> - Get the information if the pages have been written-to (PAGE_IS_Wt),
>>   file mapped (PAGE_IS_FILE), present (PAGE_IS_PRESENT) or swapped
>>   (PAGE_IS_SWAPPED).
>> - Write-protect the pages (PAGEMAP_WP_ENGAGE) to start finding which
>>   pages have been written-to.
>> - Find pages which have been written-to and write protect the pages
>>   (atomic PAGE_IS_NOT_WP + PAGEMAP_WP_ENGAGE)
>>
>> The uffd should have been registered on the memory range before performing
>> any WP/WT (Write Protect/Writtern-To) related operations with the IOCTL.
>>
>> struct pagemap_scan_args is used as the argument of the IOCTL. In this
>> struct:
>> - The range is specified through start and len.
>> - The output buffer and size is specified as vec and vec_len.
>> - The optional maximum requested pages are specified in the max_pages.
>> - The flags can be specified in the flags field. The PAGEMAP_WP_ENGAGE
>>   is the only added flag at this time.
>> - The masks are specified in required_mask, anyof_mask, excluded_ mask
>>   and return_mask.
>>
>> This IOCTL can be extended to get information about more PTE bits. This
>> patch has evolved from a basic patch from Gabriel Krisman Bertazi.
>>
>> Signed-off-by: Muhammad Usama Anjum <usama.anjum@xxxxxxxxxxxxx>
>> ---
>> Changes in v7:
>> - Rebase on top of latest next
>> - Fix some corner cases
>> - Base soft-dirty on the uffd wp async
>> - Update the terminologies
>> - Optimize the memory usage inside the ioctl
>>
>> Changes in v6:
>> - Rename variables and update comments
>> - Make IOCTL independent of soft_dirty config
>> - Change masks and bitmap type to _u64
>> - Improve code quality
>>
>> Changes in v5:
>> - Remove tlb flushing even for clear operation
>>
>> Changes in v4:
>> - Update the interface and implementation
>>
>> Changes in v3:
>> - Tighten the user-kernel interface by using explicit types and add more
>>   error checking
>>
>> Changes in v2:
>> - Convert the interface from syscall to ioctl
>> - Remove pidfd support as it doesn't make sense in ioctl
>> ---
>>  fs/proc/task_mmu.c            | 300 ++++++++++++++++++++++++++++++++++
>>  include/uapi/linux/fs.h       |  50 ++++++
>>  tools/include/uapi/linux/fs.h |  50 ++++++
>>  3 files changed, 400 insertions(+)
>>
>> diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c
>> index e35a0398db63..ba70faadf403 100644
>> --- a/fs/proc/task_mmu.c
>> +++ b/fs/proc/task_mmu.c
>> @@ -19,6 +19,7 @@
>>  #include <linux/shmem_fs.h>
>>  #include <linux/uaccess.h>
>>  #include <linux/pkeys.h>
>> +#include <linux/minmax.h>
>>  
>>  #include <asm/elf.h>
>>  #include <asm/tlb.h>
>> @@ -1135,6 +1136,22 @@ static inline void clear_soft_dirty(struct vm_area_struct *vma,
>>  }
>>  #endif
>>  
>> +static inline bool is_pte_uffd_wp(pte_t pte)
>> +{
>> +	if ((pte_present(pte) && pte_uffd_wp(pte)) ||
>> +	    (is_swap_pte(pte) && pte_swp_uffd_wp(pte)))
>> +		return true;
> 
> This is an interesting way to detect whether the page is written..  I would
> have thought you would reuse soft-dirty bit.
> 
> For swap case, you missed the pte markers (which can exist for !anon
> memory).  You can have a look at pte_swp_uffd_wp_any().
Thank you for mentioning it. I'll update in next version.

> 
>> +	return false;
>> +}
>> +
>> +static inline bool is_pmd_uffd_wp(pmd_t pmd)
>> +{
>> +	if ((pmd_present(pmd) && pmd_uffd_wp(pmd)) ||
>> +	    (is_swap_pmd(pmd) && pmd_swp_uffd_wp(pmd)))
>> +		return true;
>> +	return false;
>> +}
>> +
>>  #if defined(CONFIG_MEM_SOFT_DIRTY) && defined(CONFIG_TRANSPARENT_HUGEPAGE)
>>  static inline void clear_soft_dirty_pmd(struct vm_area_struct *vma,
>>  		unsigned long addr, pmd_t *pmdp)
>> @@ -1763,11 +1780,294 @@ static int pagemap_release(struct inode *inode, struct file *file)
>>  	return 0;
>>  }
>>  
>> +#define PAGEMAP_OP_MASK		(PAGE_IS_WT | PAGE_IS_FILE |	\
>> +				 PAGE_IS_PRESENT | PAGE_IS_SWAPPED)
>> +#define PAGEMAP_NONWT_OP_MASK	(PAGE_IS_FILE |	PAGE_IS_PRESENT | PAGE_IS_SWAPPED)
>> +#define IS_WP_ENGAGE_OP(a)	(a->flags & PAGEMAP_WP_ENGAGE)
>> +#define IS_GET_OP(a)		(a->vec)
>> +#define PAGEMAP_SCAN_BITMAP(wt, file, present, swap)	\
>> +	(wt | file << 1 | present << 2 | swap << 3)
>> +#define IS_WT_REQUIRED(a)				\
>> +	((a->required_mask & PAGE_IS_WT) ||	\
>> +	 (a->anyof_mask & PAGE_IS_WT))
>> +
>> +struct pagemap_scan_private {
>> +	struct page_region *vec;
>> +	struct page_region prev;
>> +	unsigned long vec_len, vec_index;
>> +	unsigned int max_pages, found_pages, flags;
>> +	unsigned long required_mask, anyof_mask, excluded_mask, return_mask;
>> +};
>> +
>> +static int pagemap_scan_test_walk(unsigned long start, unsigned long end, struct mm_walk *walk)
>> +{
>> +	struct pagemap_scan_private *p = walk->private;
>> +	struct vm_area_struct *vma = walk->vma;
>> +
>> +	if (IS_WT_REQUIRED(p) && !vma_can_userfault(vma, vma->vm_flags))
>> +		return -EPERM;
> 
> vma_can_userfault() is too coarse.  Maybe what you wanted to check is
> userfaultfd_wp(vma)?
> 
>> +	if (IS_GET_OP(p) && p->max_pages && (p->found_pages == p->max_pages))
>> +		return -ENOSPC;
> 
> This is the function to test "whether the walker should walk the vma
> specified".  This check should IIUC be meaningless because found_pages
> doesn't boost during vma switching, while OTOH your pmd walker fn should do
> proper check when increasing found_pages and return -ENOSPC properly when
> the same condition met.  That should be enough, IMHO.
This check is needed in case we want to abort the walk at once. We return
negative value from here which aborts the walk. Returning negative value
from pmd_entry doesn't abort the walk. So this check is needed in the
test_walk.

> 
> I saw it already a few times in this patch, I think maybe you want a helper
> for this one taking *p as argument.
> 
>> +	if (vma->vm_flags & VM_PFNMAP)
>> +		return 1;
>> +	return 0;
>> +}
>> +
>> +static inline int add_to_out(bool wt, bool file, bool pres, bool swap,
>> +			     struct pagemap_scan_private *p, unsigned long addr, unsigned int len)
>> +{
>> +	unsigned long bitmap, cur = PAGEMAP_SCAN_BITMAP(wt, file, pres, swap);
>> +	bool cpy = true;
>> +	struct page_region *prev = &p->prev;
>> +
>> +	if (p->required_mask)
>> +		cpy = ((p->required_mask & cur) == p->required_mask);
>> +	if (cpy && p->anyof_mask)
>> +		cpy = (p->anyof_mask & cur);
>> +	if (cpy && p->excluded_mask)
>> +		cpy = !(p->excluded_mask & cur);
>> +	bitmap = cur & p->return_mask;
>> +	if (cpy && bitmap) {
>> +		if ((prev->len) && (prev->bitmap == bitmap) &&
>> +		    (prev->start + prev->len * PAGE_SIZE == addr)) {
>> +			prev->len += len;
>> +			p->found_pages += len;
>> +		} else if (p->vec_index < p->vec_len) {
>> +			if (prev->len) {
>> +				memcpy(&p->vec[p->vec_index], prev, sizeof(struct page_region));
>> +				p->vec_index++;
>> +			}
>> +			prev->start = addr;
>> +			prev->len = len;
>> +			prev->bitmap = bitmap;
>> +			p->found_pages += len;
>> +		} else {
>> +			return -ENOSPC;
>> +		}
> 
> [1]
> 
>> +	}
>> +	return 0;
>> +}
>> +
>> +static inline int export_prev_to_out(struct pagemap_scan_private *p, struct page_region __user *vec,
>> +				     unsigned long *vec_index)
>> +{
>> +	struct page_region *prev = &p->prev;
>> +
>> +	if (prev->len) {
>> +		if (copy_to_user(&vec[*vec_index], prev, sizeof(struct page_region)))
>> +			return -EFAULT;
>> +		p->vec_index++;
>> +		(*vec_index)++;
>> +		prev->len = 0;
>> +	}
>> +	return 0;
>> +}
> 
> I'd rather not have this helper; it doesn't really help a lot.
It just helps in keeping do_pagemap_cmd() clean. Please let me know if I
should remove it in next revision.

> 
>> +
>> +static inline int pagemap_scan_pmd_entry(pmd_t *pmd, unsigned long addr,
>> +					 unsigned long end, struct mm_walk *walk)
>> +{
>> +	struct pagemap_scan_private *p = walk->private;
>> +	struct vm_area_struct *vma = walk->vma;
>> +	unsigned long start = addr;
>> +	unsigned int len;
>> +	spinlock_t *ptl;
>> +	int ret = 0;
>> +	pte_t *pte;
>> +	bool pmd_wt;
>> +
>> +	if ((walk->vma->vm_end < addr) || (p->max_pages && p->found_pages == p->max_pages))
>> +		return 0;
>> +	end = min(end, walk->vma->vm_end);
> 
> None of the check above is needed, I think..
Sorry, it is residue from prevision revision as VMA was being split there.
It'll be removed in next revision.

> 
> vma ranges are checked before pmd_entry() called.  found_pages should be
> checked when increased (I think it's [1] above).
Okay. I'll do it.

> 
>> +
>> +#ifdef CONFIG_TRANSPARENT_HUGEPAGE
>> +	ptl = pmd_trans_huge_lock(pmd, vma);
>> +	if (ptl) {
>> +		pmd_wt = !is_pmd_uffd_wp(*pmd);
>> +		/*
>> +		 * Break huge page into small pages if operation needs to be performed is
>> +		 * on a portion of the huge page or the return buffer cannot store complete
>> +		 * data.
>> +		 */
>> +		if (pmd_wt && (IS_WP_ENGAGE_OP(p) && (end - addr < HPAGE_SIZE))) {
>> +			spin_unlock(ptl);
>> +			split_huge_pmd(vma, pmd, addr);
>> +			goto process_smaller_pages;
>> +		}
>> +		if (IS_GET_OP(p)) {
>> +			len = (end - addr)/PAGE_SIZE;
>> +			if (p->max_pages && p->found_pages + len > p->max_pages)
>> +				len = p->max_pages - p->found_pages;
>> +
>> +			ret = add_to_out(pmd_wt, vma->vm_file, pmd_present(*pmd),
>> +					 is_swap_pmd(*pmd), p, addr, len);
>> +			if (ret) {
>> +				spin_unlock(ptl);
>> +				return ret;
>> +			}
>> +		}
>> +		spin_unlock(ptl);
>> +		if (IS_WP_ENGAGE_OP(p) && pmd_wt) {
>> +			BUG_ON(start & ~HPAGE_MASK);
>> +
>> +			ret = wp_range_async(vma, addr, HPAGE_SIZE);
>> +		}
>> +		return ret;
>> +	}
>> +process_smaller_pages:
>> +	if (pmd_trans_unstable(pmd))
>> +		return 0;
>> +#endif /* CONFIG_TRANSPARENT_HUGEPAGE */
>> +
>> +	pte = pte_offset_map_lock(vma->vm_mm, pmd, addr, &ptl);
>> +	for (; addr < end && !ret && (!p->max_pages || (p->found_pages < p->max_pages))
>> +	     ; pte++, addr += PAGE_SIZE) {
>> +		if (IS_GET_OP(p)) {
> 
> This 'if' should be out - skip loop if not needed.
Thanks for spotting it.

> 
>> +			ret = add_to_out(!is_pte_uffd_wp(*pte), vma->vm_file, pte_present(*pte),
>> +					 is_swap_pte(*pte), p, addr, 1);
>> +			if (ret)
>> +				break;
>> +		}
>> +	}
>> +	pte_unmap_unlock(pte - 1, ptl);
>> +	if (IS_WP_ENGAGE_OP(p)) {
>> +		BUG_ON(start & ~PAGE_MASK);
>> +		ret = wp_range_async(vma, start, addr - start);
>> +	}
>> +
>> +	cond_resched();
>> +	return ret;
>> +}
>> +
>> +static int pagemap_scan_pte_hole(unsigned long addr, unsigned long end, int depth,
>> +				 struct mm_walk *walk)
>> +{
>> +	struct pagemap_scan_private *p = walk->private;
>> +	struct vm_area_struct *vma = walk->vma;
>> +	unsigned int len;
>> +	int ret = 0;
>> +
>> +	if (vma) {
>> +		len = (end - addr)/PAGE_SIZE;
>> +		if (p->max_pages && p->found_pages + len > p->max_pages)
>> +			len = p->max_pages - p->found_pages;
>> +		ret = add_to_out(false, vma->vm_file, false, false, p, addr, len);
>> +	}
>> +
>> +	return ret;
>> +}
>> +
>> +static const struct mm_walk_ops pagemap_scan_ops = {
>> +	.test_walk = pagemap_scan_test_walk,
>> +	.pmd_entry = pagemap_scan_pmd_entry,
> 
> Do we care about hugetlb at all?  So far it seems you don't.  It's fine,
> then if you decided to go the uffd-wp way you can explicit declare no
> support on hugetlb, as uffd-wp sync supports it so it should be by default
> supported otherwise.
Okay. I'll mention in the commit message and in the comment here as well.

> 
>> +	.pte_hole = pagemap_scan_pte_hole,
>> +};
> 

-- 
BR,
Muhammad Usama Anjum