linux-kernel - Re: [PATCH] shmem: Update folio if shmem_replace

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Date:   Fri, 5 Aug 2022 20:34:25 +0000
From:   William Kucharski <william.kucharski@...cle.com>
To:     Matthew Wilcox <willy@...radead.org>
CC:     Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] shmem: Update folio if shmem_replace_page() updates the
 page

Looks good.

Reviewed-by: William Kucharski <william.kucharski@...cle.com>

> On Aug 2, 2022, at 21:18, Matthew Wilcox <willy@...radead.org> wrote:
> 
> On Tue, Aug 02, 2022 at 05:46:37PM -0700, Andrew Morton wrote:
>>> On Sat, 30 Jul 2022 05:25:18 +0100 "Matthew Wilcox (Oracle)" <willy@...radead.org> wrote:
>>> 
>>> If we allocate a new page, we need to make sure that our folio matches
>>> that new page.  This will be solved by changing shmem_replace_page()
>>> to shmem_replace_folio(), but this is the minimal fix.
>>> 
>>> ...
>>> 
>>> --- a/mm/shmem.c
>>> +++ b/mm/shmem.c
>>> @@ -1771,6 +1771,7 @@ static int shmem_swapin_folio(struct inode *inode, pgoff_t index,
>>> 
>>>    if (shmem_should_replace_folio(folio, gfp)) {
>>>        error = shmem_replace_page(&page, gfp, info, index);
>>> +        folio = page_folio(page);
>>>        if (error)
>>>            goto failed;
>>>    }
>> 
>> What are the user-visible runtime effects of the bug?
>> 
>> Should we backport this into 5.19.X?
> 
> Definitely should be backported.  The next line not visible in this
> patch context says:
> 
>        error = shmem_add_to_page_cache(folio, mapping, index,
>                                        swp_to_radix_entry(swap), gfp,
>                                        charge_mm);
> 
> so if we do end up in this path, we store the wrong page in the
> shmem inode's page cache, and I would rather imagine that data
> corruption ensues.
>