Re: [PATCH v2] mm: filemap: retain mapped dropbehind folios

[Tal Zussman]

On 8/29/26 6:53 PM, Tal Zussman wrote:
> On Sat, 29 Aug 2026 20:30:47 +0800, Wenjie Qi <qwjhust@xxxxxxxxx> wrote:
>> Fault-around can map ready dropbehind folios without going through the
>> normal page-cache lookup that clears dropbehind. A mapping represents a
>> competing cached user, so retain the folio instead of forcibly unmapping it
>> when writeback completes.
>>
>> Unmapped dropbehind folios continue through the existing invalidation path.
>>
>> Signed-off-by: Wenjie Qi <qiwenjie@xxxxxxxxxx>
> 
> Overall looks good to me. It may be nice to add a sentence about the
> safety aspect of this in the commit message. Maybe something like:
> 
>   folio_unmap_invalidate() may sleep if the folio is mapped, so this
>   also ensures that folio_end_dropbehind() is safe to call from
>   non-preemptible task context.
> 
> Reviewed-by: Tal Zussman <tz2294@xxxxxxxxxxxx>
> 

And maybe:

Fixes: fb7d3bc41493 ("mm/filemap: drop streaming/uncached pages when writeback completes")