linux-kernel - Re: [PATCH 1/2] mm/gup: Fix hugepd handling in hugetlb rework

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Message-ID: <19c5b260-fddf-478e-abc5-1c645957b562@redhat.com>
Date: Mon, 29 Apr 2024 09:17:44 +0200
From: David Hildenbrand <david@...hat.com>
To: Peter Xu <peterx@...hat.com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org
Cc: Christophe Leroy <christophe.leroy@...roup.eu>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Aneesh Kumar K . V" <aneesh.kumar@...nel.org>,
 Lorenzo Stoakes <lstoakes@...il.com>, John Hubbard <jhubbard@...dia.com>,
 linuxppc-dev@...ts.ozlabs.org, Muchun Song <muchun.song@...ux.dev>,
 Jason Gunthorpe <jgg@...dia.com>
Subject: Re: [PATCH 1/2] mm/gup: Fix hugepd handling in hugetlb rework

On 28.04.24 21:01, Peter Xu wrote:
> Commit a12083d721d7 added hugepd handling for gup-slow, reusing gup-fast
> functions.  follow_hugepd() correctly took the vma pointer in, however
> didn't pass it over into the lower functions, which was overlooked.
> 
> The issue is gup_fast_hugepte() uses the vma pointer to make the correct
> decision on whether an unshare is needed for a FOLL_PIN|FOLL_LONGTERM.  Now
> without vma ponter it will constantly return "true" (needs an unshare) for
> a page cache, even though in the SHARED case it will be wrong to unshare.
> 
> The other problem is, even if an unshare is needed, it now returns 0 rather
> than -EMLINK, which will not trigger a follow up FAULT_FLAG_UNSHARE fault.
> That will need to be fixed too when the unshare is wanted.
> 
> gup_longterm test didn't expose this issue in the past because it didn't
> yet test R/O unshare in this case, another separate patch will enable that
> in future tests.
> 
> Fix it by passing vma correctly to the bottom, rename gup_fast_hugepte()
> back to gup_hugepte() as it is shared between the fast/slow paths, and also
> allow -EMLINK to be returned properly by gup_hugepte() even though gup-fast
> will take it the same as zero.
> 
> Reported-by: David Hildenbrand <david@...hat.com>
> Fixes: a12083d721d7 ("mm/gup: handle hugepd for follow_page()")
> Signed-off-by: Peter Xu <peterx@...hat.com>
> ---

LGTM

Reviewed-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb