Re: [PATCH v1 09/14] mm/mshare: Do not free PTEs for mshare'd PTEs
From: Khalid Aziz
Date: Wed Jul 06 2022 - 16:34:15 EST
On 7/3/22 14:54, Andy Lutomirski wrote:
On 6/29/22 10:38, Khalid Aziz wrote:
On 5/30/22 22:24, Barry Song wrote:
On Tue, Apr 12, 2022 at 4:07 AM Khalid Aziz <khalid.aziz@xxxxxxxxxx> wrote:
mshare'd PTEs should not be removed when a task exits. These PTEs
are removed when the last task sharing the PTEs exits. Add a check
for shared PTEs and skip them.
Signed-off-by: Khalid Aziz <khalid.aziz@xxxxxxxxxx>
---
mm/memory.c | 22 +++++++++++++++++++---
1 file changed, 19 insertions(+), 3 deletions(-)
diff --git a/mm/memory.c b/mm/memory.c
index c77c0d643ea8..e7c5bc6f8836 100644
--- a/mm/memory.c
+++ b/mm/memory.c
@@ -419,16 +419,25 @@ void free_pgtables(struct mmu_gather *tlb, struct vm_area_struct *vma,
} else {
/*
* Optimization: gather nearby vmas into one call down
+ * as long as they all belong to the same mm (that
+ * may not be the case if a vma is part of mshare'd
+ * range
*/
while (next && next->vm_start <= vma->vm_end + PMD_SIZE
- && !is_vm_hugetlb_page(next)) {
+ && !is_vm_hugetlb_page(next)
+ && vma->vm_mm == tlb->mm) {
vma = next;
next = vma->vm_next;
unlink_anon_vmas(vma);
unlink_file_vma(vma);
}
- free_pgd_range(tlb, addr, vma->vm_end,
- floor, next ? next->vm_start : ceiling);
+ /*
+ * Free pgd only if pgd is not allocated for an
+ * mshare'd range
+ */
+ if (vma->vm_mm == tlb->mm)
+ free_pgd_range(tlb, addr, vma->vm_end,
+ floor, next ? next->vm_start : ceiling);
}
vma = next;
}
@@ -1551,6 +1560,13 @@ void unmap_page_range(struct mmu_gather *tlb,
pgd_t *pgd;
unsigned long next;
+ /*
+ * If this is an mshare'd page, do not unmap it since it might
+ * still be in use.
+ */
+ if (vma->vm_mm != tlb->mm)
+ return;
+
expect unmap, have you ever tested reverse mapping in vmscan, especially
folio_referenced()? are all vmas in those processes sharing page table still
in the rmap of the shared page?
without shared PTE, if 1000 processes share one page, we are reading 1000
PTEs, with it, are we reading just one? or are we reading the same PTE
1000 times? Have you tested it?
We are treating mshared region same as threads sharing address space. There is one PTE that is being used by all
processes and the VMA maintained in the separate mshare mm struct that also holds the shared PTE is the one that gets
added to rmap. This is a different model with mshare in that it adds an mm struct that is separate from the mm structs
of the processes that refer to the vma and pte in mshare mm struct. Do you see issues with rmap in this model?
I think this patch is actually the most interesting bit of the series by far. Most of the rest is defining an API
(which is important!) and figuring out semantics. This patch changes something rather fundamental about how user
address spaces work: what vmas live in them. So let's figure out its effects.
I admit I'm rather puzzled about what vm_mm is for in the first place. In current kernels (without your patch), I think
it's a pretty hard requirement for vm_mm to equal the mm for all vmas in an mm. After a quick and incomplete survey,
vm_mm seems to be mostly used as a somewhat lazy way to find the mm. Let's see:
file_operations->mmap doesn't receive an mm_struct. Instead it infers the mm from vm_mm. (Why? I don't know.)
Some walk_page_range users seem to dig the mm out of vm_mm instead of mm_walk.
Some manual address space walkers start with an mm, don't bother passing it around, and dig it back out of of vm_mm.
For example, unuse_vma() and all its helpers.
The only real exception I've found so far is rmap: AFAICS (on quick inspection -- I could be wrong), rmap can map from a
folio to a bunch of vmas, and the vmas' mms are not stored separately but instead determined by vm_mm.
Your patch makes me quite nervous. You're potentially breaking any kernel code path that assumes that mms only contain
vmas that have vm_mm == mm. And you're potentially causing rmap to be quite confused. I think that if you're going to
take this approach, you need to clearly define the new semantics of vm_mm and audit or clean up every user of vm_mm in
the tree. This may be nontrivial (especially rmap), although a cleanup of everything else to stop using vm_mm might be
valuable.
But I'm wondering if it would be better to attack this from a different direction. Right now, there's a hardcoded
assumption that an mm owns every page table it references. That's really the thing you're changing. To me, it seems
that a magical vma that shares page tables should still be a vma that belongs to its mm_struct -- munmap() and
potentialy other m***() operations should all work on it, existing find_vma() users should work, etc.
So maybe instead there should be new behavior (by a VM_ flag or otherwise) that indicates that a vma owns its PTEs. It
could even be a vm_operation, although if anyone ever wants regular file mappings to share PTEs, then a vm_operation
doesn't really make sense.
Hi Andy,
You are absolutely right. Dragons lie on the path to changing the sense of vm_mm. Dave H pointed out potential issues
with rb_tree as well. As I have looked at more code, it seems breaking the assumption that vm_mm always points to
containing mm struct opens up the possibility of code breaking in strange ways in odd places. As a result, I have
changed the code in v2 patches to not break this assumption about vm_mm and instead I rewrote the code to use the vm
flag VM_SHARED_PT and vm_private_data everywhere it needed to find the mshare mm struct. All the vmas belonging to the
new mm struct for mshare region also have their vm_mm pointing to the mshare mm_struct and that keeps all vma operations
working normally.
Thanks,
Khalid