linux-kernel - Re: [PATCH] afs: Fix waiting for writeback then skipping folio

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Message-Id: <20230608175031.9c534e7f554de89e3d972ab2@linux-foundation.org>
Date:   Thu, 8 Jun 2023 17:50:31 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Vishal Moola (Oracle)" <vishal.moola@...il.com>
Cc:     linux-afs@...ts.infradead.org, linux-kernel@...r.kernel.org,
        dhowells@...hat.com
Subject: Re: [PATCH] afs: Fix waiting for writeback then skipping folio

On Wed,  7 Jun 2023 13:41:20 -0700 "Vishal Moola (Oracle)" <vishal.moola@...il.com> wrote:

> Commit acc8d8588cb7 converted afs_writepages_region() to write back a
> folio batch. The function waits for writeback to a folio, but then
> proceeds to the rest of the batch without trying to write that folio
> again. This patch fixes has it attempt to write the folio again.
> 
> This has only been compile tested.

This seems fairly serious?

> --- a/fs/afs/write.c
> +++ b/fs/afs/write.c
> @@ -731,6 +731,7 @@ static int afs_writepages_region(struct address_space *mapping,
>  			 * (changing page->mapping to NULL), or even swizzled
>  			 * back from swapper_space to tmpfs file mapping
>  			 */
> +try_again:
>  			if (wbc->sync_mode != WB_SYNC_NONE) {
>  				ret = folio_lock_killable(folio);
>  				if (ret < 0) {
> @@ -757,6 +758,7 @@ static int afs_writepages_region(struct address_space *mapping,
>  #ifdef CONFIG_AFS_FSCACHE
>  					folio_wait_fscache(folio);
>  #endif
> +					goto try_again;
>  				} else {
>  					start += folio_size(folio);
>  				}

>From my reading, we'll fail to write out the dirty data.  Presumably
not easily observable, as it will get written out again later on.  But
we're also calling afs_write_back_from_locked_folio() with an unlocked
folio, which might cause mayhem.

So I'm suspecting that a cc:stable is needed.  David, could you please
take a look and perhaps retest?

Thanks.