Re: [PATCH] mm: shmem: fix potential data corruption during shmem swapin

[Kairui Song <ryncsn@gmail.com>]

On Mon, Feb 24, 2025 at 4:47 PM Baolin Wang
<baolin.wang@linux.alibaba.com> wrote:
>
> Alex and Kairui reported some issues (system hang or data corruption) when
> swapping out or swapping in large shmem folios. This is especially easy to
> reproduce when the tmpfs is mount with the 'huge=within_size' parameter.
> Thanks to Kairui's reproducer, the issue can be easily replicated.
>
> The root cause of the problem is that swap readahead may asynchronously
> swap in order 0 folios into the swap cache, while the shmem mapping can
> still store large swap entries. Then an order 0 folio is inserted into
> the shmem mapping without splitting the large swap entry, which overwrites
> the original large swap entry, leading to data corruption.
>
> When getting a folio from the swap cache, we should split the large swap
> entry stored in the shmem mapping if the orders do not match, to fix this
> issue.
>
> Fixes: 809bc86517cc ("mm: shmem: support large folio swap out")
> Reported-by: Alex Xu (Hello71) <alex_y_xu@yahoo.ca>
> Reported-by: Kairui Song <ryncsn@gmail.com>

Maybe you can add a Closes:?

> Signed-off-by: Baolin Wang <baolin.wang@linux.alibaba.com>
> ---
>  mm/shmem.c | 31 +++++++++++++++++++++++++++----
>  1 file changed, 27 insertions(+), 4 deletions(-)
>
> diff --git a/mm/shmem.c b/mm/shmem.c
> index 4ea6109a8043..cebbac97a221 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -2253,7 +2253,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
>         struct folio *folio = NULL;
>         bool skip_swapcache = false;
>         swp_entry_t swap;
> -       int error, nr_pages;
> +       int error, nr_pages, order, split_order;
>
>         VM_BUG_ON(!*foliop || !xa_is_value(*foliop));
>         swap = radix_to_swp_entry(*foliop);
> @@ -2272,10 +2272,9 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
>
>         /* Look it up and read it in.. */
>         folio = swap_cache_get_folio(swap, NULL, 0);
> +       order = xa_get_order(&mapping->i_pages, index);
>         if (!folio) {
> -               int order = xa_get_order(&mapping->i_pages, index);
>                 bool fallback_order0 = false;
> -               int split_order;
>
>                 /* Or update major stats only when swapin succeeds?? */
>                 if (fault_type) {
> @@ -2339,6 +2338,29 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
>                         error = -ENOMEM;
>                         goto failed;
>                 }
> +       } else if (order != folio_order(folio)) {
> +               /*
> +                * Swap readahead may swap in order 0 folios into swapcache
> +                * asynchronously, while the shmem mapping can still stores
> +                * large swap entries. In such cases, we should split the
> +                * large swap entry to prevent possible data corruption.
> +                */
> +               split_order = shmem_split_large_entry(inode, index, swap, gfp);
> +               if (split_order < 0) {
> +                       error = split_order;
> +                       goto failed;
> +               }
> +
> +               /*
> +                * If the large swap entry has already been split, it is
> +                * necessary to recalculate the new swap entry based on
> +                * the old order alignment.
> +                */
> +               if (split_order > 0) {
> +                       pgoff_t offset = index - round_down(index, 1 << split_order);
> +
> +                       swap = swp_entry(swp_type(swap), swp_offset(swap) + offset);
> +               }
>         }
>
>  alloced:
> @@ -2346,7 +2368,8 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
>         folio_lock(folio);
>         if ((!skip_swapcache && !folio_test_swapcache(folio)) ||
>             folio->swap.val != swap.val ||
> -           !shmem_confirm_swap(mapping, index, swap)) {
> +           !shmem_confirm_swap(mapping, index, swap) ||
> +           xa_get_order(&mapping->i_pages, index) != folio_order(folio)) {
>                 error = -EEXIST;
>                 goto unlock;
>         }
> --
> 2.43.5
>

Thanks for the fix, it works for me.

Tested-by: Kairui Song <kasong@tencent.com>