Re: [PATCH for 6.1 regression] mm, mremap: fix mremap() expanding for vma's with vm_ops->close()

[Vlastimil Babka]

On 1/19/23 14:37, Linux kernel regression tracking (Thorsten Leemhuis) wrote:
> On 17.01.23 11:19, Vlastimil Babka wrote:
>> Fabian has reported another regression in 6.1 due to ca3d76b0aa80 ("mm:
>> add merging after mremap resize"). The problem is that vma_merge() can
>> fail when vma has a vm_ops->close() method, causing is_mergeable_vma()
>> test to be negative. This was happening for vma mapping a file from
>> fuse-overlayfs, which does have the method. But when we are simply
>> expanding the vma, we never remove it due to the "merge" with the added
>> area, so the test should not prevent the expansion.
>> 
>> As a quick fix, check for such vmas and expand them using vma_adjust()
>> directly as was done before commit ca3d76b0aa80. For a more robust long
>> term solution we should try to limit the check for vma_ops->close only
>> to cases that actually result in vma removal, so that no merge would be
>> prevented unnecessarily.
>> 
>> Reported-by: Fabian Vogt <fvogt@xxxxxxxx>
>> Link: https://bugzilla.suse.com/show_bug.cgi?id=1206359#c35
>> Fixes: ca3d76b0aa80 ("mm: add merging after mremap resize")
>> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
>> Cc: Jakub Matěna <matenajakub@xxxxxxxxx>
>> Cc: <stable@xxxxxxxxxxxxxxx>
>> Tested-by: Fabian Vogt <fvogt@xxxxxxxx>
>> ---
> 
> Thx for highlighting it and CCing me.
> 
> Quick question: how fast do you think this should head towards mainline?
> 
> The patch landed in next today, so that step in the process is already
> covered. But is the issue serious enough to say "send this to Linus
> after it was a day or two in next, so it can be quickly backported to
> stable"?

I think it's not as serious as the previous one, the conditions should be
more rare. But you made me realize I should probably reply to the "stalls in
qemu" one in that sense. Thanks!

>> Thorsten: this should be added to the previous regression which wasn't
>> fully fixed by the previous patch:
>> https://linux-regtracking.leemhuis.info/regzbot/regression/20221216163227.24648-1-vbabka@xxxxxxx/
>>  mm/mremap.c | 13 ++++++++++++-
>>  1 file changed, 12 insertions(+), 1 deletion(-)
>> [...]
> 
> In that case let me just briefly drop a link to the regression, as
> regzbot will notice that and file is as an activity.
> 
> https://lore.kernel.org/lkml/20221216163227.24648-1-vbabka@xxxxxxx/
> 
> And simply consider your patch submission as a new report I track
> separately:
> 
> #regzbot introduced ca3d76b0aa80 ^
> https://bugzilla.suse.com/show_bug.cgi?id=1206359#c35
> #regzbot title mm, mremap: another issue with mremap not fully fixed
> with the previous fix for the regression
> #regzbot fix: mm, mremap: fix mremap() expanding for vma's with
> vm_ops->close()
> #regzbot ignore-activity
> 
> Not ideal, but that will make sure it's on regzbot radar (where way too
> many dots appear currently, as I'm a bit behind with things... :-/ )
> 
> Ciao, Thorsten (wearing his 'the Linux kernel's regression tracker' hat)
> --
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