Re: [PATCH 1/2] mm, thp: check page mapping when truncating page cache

[Yang Shi]

On Tue, Sep 7, 2021 at 7:41 PM Rongwei Wang
<rongwei.wang@xxxxxxxxxxxxxxxxx> wrote:
>
>
>
> On Sep 8, 2021, at 2:08 AM, Yang Shi <shy828301@xxxxxxxxx> wrote:
>
> On Mon, Sep 6, 2021 at 5:12 AM Rongwei Wang
> <rongwei.wang@xxxxxxxxxxxxxxxxx> wrote:
>
>
> Transparent huge page has supported read-only non-shmem files. The file-
> backed THP is collapsed by khugepaged and truncated when written (for
> shared libraries).
>
> However, there is race in two possible places.
>
> 1) multiple writers truncate the same page cache concurrently;
> 2) collapse_file rolls back when writer truncates the page cache;
>
> In both cases, subpage(s) of file THP can be revealed by find_get_entry
> in truncate_inode_pages_range, which will trigger PageTail BUG_ON in
> truncate_inode_page, as follows.
>
> [40326.247034] page:000000009e420ff2 refcount:1 mapcount:0 mapping:0000000000000000 index:0x7ff pfn:0x50c3ff
> [40326.247041] head:0000000075ff816d order:9 compound_mapcount:0 compound_pincount:0
> [40326.247046] flags: 0x37fffe0000010815(locked|uptodate|lru|arch_1|head)
> [40326.247051] raw: 37fffe0000000000 fffffe0013108001 dead000000000122 dead000000000400
> [40326.247053] raw: 0000000000000001 0000000000000000 00000000ffffffff 0000000000000000
> [40326.247055] head: 37fffe0000010815 fffffe001066bd48 ffff000404183c20 0000000000000000
> [40326.247057] head: 0000000000000600 0000000000000000 00000001ffffffff ffff000c0345a000
> [40326.247058] page dumped because: VM_BUG_ON_PAGE(PageTail(page))
> [40326.247077] ------------[ cut here ]------------
> [40326.247080] kernel BUG at mm/truncate.c:213!
> [40326.280581] Internal error: Oops - BUG: 0 [#1] SMP
> [40326.281077] Modules linked in: xfs(E) libcrc32c(E) rfkill(E) aes_ce_blk(E) crypto_simd(E) cryptd(E) aes_ce_cipher(E) crct10dif_ce(E) ghash_ce(E) sha1_ce(E) uio_pdrv_genirq(E) uio(E) nfsd(E) vfat(E) fat(E) auth_rpcgss(E) nfs_acl(E) lockd(E) grace(E) sunrpc(E) sch_fq_codel(E) ip_tables(E) ext4(E) mbcache(E) jbd2(E) virtio_net(E) net_failover(E) virtio_blk(E) failover(E) sha2_ce(E) sha256_arm64(E) virtio_mmio(E) virtio_pci(E) virtio_ring(E) virtio(E)
> [40326.285130] CPU: 14 PID: 11394 Comm: check_madvise_d Kdump: loaded Tainted: G        W   E     5.10.46-hugetext+ #55
> [40326.286202] Hardware name: Alibaba Cloud Alibaba Cloud ECS, BIOS 0.0.0 02/06/2015
> [40326.286968] pstate: 60400005 (nZCv daif +PAN -UAO -TCO BTYPE=--)
> [40326.287584] pc : truncate_inode_page+0x64/0x70
> [40326.288040] lr : truncate_inode_page+0x64/0x70
> [40326.288498] sp : ffff80001b60b900
> [40326.288837] x29: ffff80001b60b900 x28: 00000000000007ff
> [40326.289377] x27: ffff80001b60b9a0 x26: 0000000000000000
> [40326.289943] x25: 000000000000000f x24: ffff80001b60b9a0
> [40326.290485] x23: ffff80001b60ba18 x22: ffff0001e0999ea8
> [40326.291027] x21: ffff0000c21db300 x20: ffffffffffffffff
> [40326.291566] x19: fffffe001310ffc0 x18: 0000000000000020
> [40326.292106] x17: 0000000000000000 x16: 0000000000000000
> [40326.292655] x15: ffff0000c21db960 x14: 3030306666666620
> [40326.293197] x13: 6666666666666666 x12: 3130303030303030
> [40326.293746] x11: ffff8000117b69b8 x10: 00000000ffff8000
> [40326.294313] x9 : ffff80001012690c x8 : 0000000000000000
> [40326.294851] x7 : ffff8000114f69b8 x6 : 0000000000017ffd
> [40326.295392] x5 : ffff0007fffbcbc8 x4 : ffff80001b60b5c0
> [40326.295942] x3 : 0000000000000001 x2 : 0000000000000000
> [40326.296497] x1 : 0000000000000000 x0 : 0000000000000000
> [40326.297047] Call trace:
> [40326.297304]  truncate_inode_page+0x64/0x70
> [40326.297724]  truncate_inode_pages_range+0x550/0x7e4
> [40326.298251]  truncate_pagecache+0x58/0x80
> [40326.298662]  do_dentry_open+0x1e4/0x3c0
> [40326.299052]  vfs_open+0x38/0x44
> [40326.299377]  do_open+0x1f0/0x310
> [40326.299709]  path_openat+0x114/0x1dc
> [40326.300077]  do_filp_open+0x84/0x134
> [40326.300444]  do_sys_openat2+0xbc/0x164
> [40326.300825]  __arm64_sys_openat+0x74/0xc0
> [40326.301236]  el0_svc_common.constprop.0+0x88/0x220
> [40326.301723]  do_el0_svc+0x30/0xa0
> [40326.302089]  el0_svc+0x20/0x30
> [40326.302404]  el0_sync_handler+0x1a4/0x1b0
> [40326.302814]  el0_sync+0x180/0x1c0
> [40326.303157] Code: aa0103e0 900061e1 910ec021 9400d300 (d4210000)
> [40326.303775] ---[ end trace f70cdb42cb7c2d42 ]---
> [40326.304244] Kernel panic - not syncing: Oops - BUG: Fatal exception
>
> This checks the page mapping and retries when subpage of file THP is
> found, in truncate_inode_pages_range.
>
> Fixes: eb6ecbed0aa2 ("mm, thp: relax the VM_DENYWRITE constraint on file-backed THPs")
> Signed-off-by: Xu Yu <xuyu@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Rongwei Wang <rongwei.wang@xxxxxxxxxxxxxxxxx>
> ---
> mm/filemap.c  |  7 ++++++-
> mm/truncate.c | 17 ++++++++++++++++-
> 2 files changed, 22 insertions(+), 2 deletions(-)
>
> diff --git a/mm/filemap.c b/mm/filemap.c
> index dae481293..a3af2ec 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2093,7 +2093,6 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
>                if (!xa_is_value(page)) {
>                        if (page->index < start)
>                                goto put;
> -                       VM_BUG_ON_PAGE(page->index != xas.xa_index, page);
>                        if (page->index + thp_nr_pages(page) - 1 > end)
>                                goto put;
>                        if (!trylock_page(page))
> @@ -2102,6 +2101,12 @@ unsigned find_lock_entries(struct address_space *mapping, pgoff_t start,
>                                goto unlock;
>                        VM_BUG_ON_PAGE(!thp_contains(page, xas.xa_index),
>                                        page);
> +                       /*
> +                        * We can find and get head page of file THP with
> +                        * non-head index. The head page should have already
> +                        * be truncated with page->mapping reset to NULL.
> +                        */
> +                       VM_BUG_ON_PAGE(page->index != xas.xa_index, page);
>                }
>                indices[pvec->nr] = xas.xa_index;
>                if (!pagevec_add(pvec, page))
> diff --git a/mm/truncate.c b/mm/truncate.c
> index 714eaf1..8c59c00 100644
> --- a/mm/truncate.c
> +++ b/mm/truncate.c
> @@ -319,7 +319,8 @@ void truncate_inode_pages_range(struct address_space *mapping,
>        index = start;
>        while (index < end && find_lock_entries(mapping, index, end - 1,
>                        &pvec, indices)) {
> -               index = indices[pagevec_count(&pvec) - 1] + 1;
> +               index = indices[pagevec_count(&pvec) - 1] +
> +                       thp_nr_pages(pvec.pages[pagevec_count(&pvec) - 1]);
>
>
> I don't quite get what this change is doing for. IIUC
> find_lock_entries() already handles index advance correctly. If
> truncate range is partial THP, it will be handled in the second pass.
>
> Yes, agree.
>
> AFAICT, the problem is why the THP is not split if it will get
>
> Yes, agree.
>
> This change is not necessary, but nice to have. Because find_lock_entries()
> return only head page if any, and it is better to advance the index by thp_nr_pages(), instead of 1.
>
> If you think it introduces unnecessary complexity, I don't mind discarding this change.

IIUC this change may reduce some runtime overhead (-1 call to
find_lock_entries()), but I'd suggest you wait for the comments from
Matthew.

>
> partially truncated. Did I miss something?
>
>
>
>                truncate_exceptional_pvec_entries(mapping, &pvec, indices);
>                for (i = 0; i < pagevec_count(&pvec); i++)
>                        truncate_cleanup_page(pvec.pages[i]);
> @@ -391,6 +392,20 @@ void truncate_inode_pages_range(struct address_space *mapping,
>                        if (xa_is_value(page))
>                                continue;
>
> +                       /*
> +                        * Already truncated? We can find and get subpage
> +                        * of file THP, of which the head page is truncated.
> +                        *
> +                        * In addition, another race will be avoided, where
> +                        * collapse_file rolls back when writer truncates the
> +                        * page cache.
> +                        */
> +                       if (page_mapping(page) != mapping) {
> +                               /* Restart to make sure all gone */
> +                               index = start - 1;
> +                               continue;
> +                       }
> +
>                        lock_page(page);
>                        WARN_ON(page_to_index(page) != index);
>                        wait_on_page_writeback(page);
> —
> 1.8.3.1
>
> Thanks,
> Rongwei Wang
>