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

[Kefeng Wang <wangkefeng.wang@huawei.com>]

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.