linux-kernel - Re: [PATCH 2/5] mm,hugetlb: Document the reason to lock the folio in the faulting path

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Message-ID: <aE075ld-fOyMipcJ@localhost.localdomain>
Date: Sat, 14 Jun 2025 11:07:50 +0200
From: Oscar Salvador <osalvador@...e.de>
To: David Hildenbrand <david@...hat.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Muchun Song <muchun.song@...ux.dev>,
	James Houghton <jthoughton@...gle.com>,
	Peter Xu <peterx@...hat.com>, Gavin Guo <gavinguo@...lia.com>,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/5] mm,hugetlb: Document the reason to lock the folio in
 the faulting path

On Fri, Jun 13, 2025 at 11:47:50PM +0200, Oscar Salvador wrote:
> Maybe it's because it's Friday, but I'm confused as to why 
> do_pte_missing()->do_fault()->do_cow_fault() holds the lock while do_wp_page() doesn't
> although it might the file's page we have to copy. 

Scratch that, I see my confusion.
The first time we map the file privately, the folio must remain stable.
But if we already mapped it privately before (R/O), and we write fault on it,
we don't need to be stable (e.g: uptodated).

But I think my comment on hugetlb_no_page() still holds, because

hugetlb_fault->hugetlb_no_page->hugetlb_wp 

would be similar to do_pte_missing->do_cow, and in do_cow we hold both
the reference and the lock.
Were we might not need the lock is in hugetlb_fault->hugetlb_wp, which
would be similar to do_wp_page()->wp_page_copy.
Of course we will need to take it if it is an anonymous folio because we need
to check the re-use case.

So, it gets complicated because hugetlb_no_page() needs to call
hugetlb_wp() with the lock held in case it is a pagecache folio, and
and the same time hugetlb_wp() needs to take the lock if it us an anonymous
one for the re-use case.
So, all in all, I think that it is easier when both callers of hugetlb_wp()
hold the lock, so we do not have to do weird dances, and document why it is done.

-- 
Oscar Salvador
SUSE Labs