Re: [RFC PATCH] mm/shmem: Fix undo range for failed fallocate

[Matthew Wilcox <willy@infradead.org>]

On Tue, Nov 01, 2022 at 11:22:48AM +0800, Rui Wang wrote:
> This patch fixes data loss caused by the fallocate system
> call interrupted by a signal.
> 
> Bug: https://lore.kernel.org/linux-mm/33b85d82.7764.1842e9ab207.Coremail.chenguoqic@163.com/
> Fixes: b9a8a4195c7d ("truncate,shmem: Handle truncates that split large folios")

How does that commit introduce this bug?

> Reported-by: Guoqi Chen <chenguoqic@163.com>
> Cc: Huacai Chen <chenhuacai@loongson.cn>
> Signed-off-by: Rui Wang <kernel@hev.cc>
> ---
>  mm/shmem.c | 20 ++++++++++++--------
>  1 file changed, 12 insertions(+), 8 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index bc9b84602eec..8c8dce34eafc 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -948,11 +948,13 @@ static void shmem_undo_range(struct inode *inode, loff_t lstart, loff_t lend,
>  	folio = shmem_get_partial_folio(inode, lstart >> PAGE_SHIFT);
>  	if (folio) {
>  		same_folio = lend < folio_pos(folio) + folio_size(folio);
> -		folio_mark_dirty(folio);
> -		if (!truncate_inode_partial_folio(folio, lstart, lend)) {
> -			start = folio->index + folio_nr_pages(folio);
> -			if (same_folio)
> -				end = folio->index;
> +		if (!unfalloc || !folio_test_uptodate(folio)) {
> +			folio_mark_dirty(folio);
> +			if (!truncate_inode_partial_folio(folio, lstart, lend)) {
> +				start = folio->index + folio_nr_pages(folio);
> +				if (same_folio)
> +					end = folio->index;
> +			}

... so what you're saying is that if we allocate a page, but zeroing
it is interrupted by a signal, we cannot now remove that page from
the cache?  That seems wrong.

Surely the right solution is to remove this page from the cache if we're
interrupted by a signal.