Re: [PATCH -next] mm: don't try THP align for FS without get_unmapped_area

[Vlastimil Babka <vbabka@suse.cz>]

On 12/9/24 06:00, Kefeng Wang wrote:
> 
> 
> On 2024/12/7 14:34, Andrew Morton wrote:
>> On Fri, 6 Dec 2024 15:03:45 +0800 Kefeng Wang <wangkefeng.wang@huawei.com> wrote:
>> 
>>> Commit ed48e87c7df3 ("thp: add thp_get_unmapped_area_vmflags()")
>>> changes thp_get_unmapped_area() to thp_get_unmapped_area_vmflags()
>>> in __get_unmapped_area(), which won't setup get_area for anonymous
>>> mappings, but it leads to always try THP align when file ops without
>>> '.get_unmapped_area' callback too as the get_area is NULL.
>>>
>>> Since commit efa7df3e3bb5 ("mm: align larger anonymous mappings on
>>> THP boundaries") only want to enable THP align for anonymous, adding
>>> !file check to fix it.
>> 
>> The above is tough.  I attempted a rewrite, please review for accuracy
>> and completeness:
> 
> Forgive my English, thanks for rewriting the better changelog.
>> 
>> : Commit ed48e87c7df3 ("thp: add thp_get_unmapped_area_vmflags()") changes
>> : thp_get_unmapped_area() to thp_get_unmapped_area_vmflags() in
>> : __get_unmapped_area(), which doesn't initialize local get_area for
>> : anonymous mappings.  This leads to us always trying THP alignment even for
>> : file_operations which have a NULL ->get_unmapped_area() callback.
>> :
>> : Since commit efa7df3e3bb5 ("mm: align larger anonymous mappings on THP
>> : boundaries") we only want to enable THP alignment for anonymous mappings,
>> : so add a !file check to avoid attempting THP alignment for file mappings.
>> 
>> Also, the changelog failed to describe the userspace-visible effects of
>> the flaw, which is basically essential when fixing bugs.
>> 
>> The bug has been there since 6.10 so it would be interesting to learn
>> why it took this long to be noticed.
> 
> Found issue by code inspection. THP alignment is used for easy or more
> pmd mappings, from vma side, I don't think it will introduce usespace-
> visible effects, only different vma address, but I don't know if there's
> any other effect.

How about:

This may cause unnecessary VMA fragmentation and potentially worse
performance on filesystems that do not actually support THPs and thus cannot
benefit from the alignment.