Re: [PATCH] mm/hugetlb: Move vmf_anon_prepare upfront in hugetlb_wp

[David Hildenbrand <dhildenb@redhat.com>]

Am 27.05.24 um 10:53 schrieb Oscar Salvador:
> On Tue, May 21, 2024 at 09:34:46AM +0200, Oscar Salvador wrote:
>> I did not hit this bug, I just spotted this because I was looking at hugetlb_wp
>> for some other reason. And I did not want to get creative to see if I could
>> trigger this so I could get a backtrace.
>> My assumption is that we could trigger this if 1) this was a shared mapping,
>> so no anon_vma and 2) we call in GUP code with FOLL_WRITE, which would cause
>> the FLAG_UNSHARE to be passed, so we will end up in hugetlb_wp().
> 
> So I checked this again and I have to confess I am bit confused.
> 
> hugetlb_wp() can be called from either hugetlb_fault() or hugetlb_no_page().
> 
> hugetlb_fault()->hugetlb_wp() upon FAULT_FLAG_{WRITE,UNSHARE}
> hugetlb_no_page->hugetlb_wp()-> upon FAULT_FLAG_WRITE && !VM_SHARED
> 
> hugetlb_no_page()->vmf_anon_prepare() upon !VM_SHARED, which means that VM_SHARED
> mappings do not have vma->anon_vma, while others do.
> 
> hugetlb_wp() will call set_huge_ptep_writable() right away and return if it sees
> that the mapping is shared.
> So the only other we have to end up in hugetlb_wp() is via FAULT_FLAG_UNSHARE.
> For that to happen gup_must_unshare() must return true, which means the following
> assumptions must hold.
> 
> - For Anonymous pages:
>    1) !PageAnonExclusive
> - For Filebacked pages:
>    2) We do not have a vma
>    3) It is a COW mapping
> 
> 1) If gup_must_unshare() returns true for Anonymous pages because the page is not
>     exclusive and must be unshared, hugetlb_wp() will already see the
>     vma->anon_prepare being initialized because of the previous
>     hugetlb_no_page()->vmf_anon_prepare.
> 
> 2) I do not quite understand this case.

gup_must_unshare() without a VMA is only used for GUP-fast. Before triggering a 
page fault we always fallback to GUP-slow first, where we have a VMA.

IMHO we better not make assumptions that hugetlb_wp() will always already have 
an anon VMA.

-- 
Thanks,

David / dhildenb