linux-kernel - Re: [PATCH v2 3/7] mm/hugetlb: Rename avoid_reserve to cow_from

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Message-ID: <Z4T3Ap7M-Jw2KJZ7@localhost.localdomain>
Date: Mon, 13 Jan 2025 12:20:34 +0100
From: Oscar Salvador <osalvador@...e.de>
To: Peter Xu <peterx@...hat.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	Breno Leitao <leitao@...ian.org>, Rik van Riel <riel@...riel.com>,
	Muchun Song <muchun.song@...ux.dev>,
	Naoya Horiguchi <nao.horiguchi@...il.com>,
	Roman Gushchin <roman.gushchin@...ux.dev>,
	Ackerley Tng <ackerleytng@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v2 3/7] mm/hugetlb: Rename avoid_reserve to cow_from_owner

On Tue, Jan 07, 2025 at 03:39:58PM -0500, Peter Xu wrote:
> The old name "avoid_reserve" can be too generic and can be used wrongly in
> the new call sites that want to allocate a hugetlb folio.
> 
> It's confusing on two things: (1) whether one can opt-in to avoid global
> reservation, and (2) whether it should take more than one count.
> 
> In reality, this flag is only used in an extremely hacky path, in an
> extremely hacky way in hugetlb CoW path only, and always use with 1 saying
> "skip global reservation".  Rename the flag to avoid future abuse of this
> flag, making it a boolean so as to reflect its true representation that
> it's not a counter.  To make it even harder to abuse, add a comment above
> the function to explain it.
> 
> Signed-off-by: Peter Xu <peterx@...hat.com>

I agree that the current name is quite misleading, and this patch
improves the situation substantially.
The only thing I am missing here is that the comment you added could be
more explanatory as to why new call sites do not want to make use of the
flag.

IIRC, not using so, will bypass all vma level reservations as you
mentioned, which means that the child can get killed if the parent
makes use of the page, as it is the parent the only one that made a
reservation.

So maybe dropping a hint would be nice.

Reviewed-by: Oscar Salvador <osalvador@...e.de>


-- 
Oscar Salvador
SUSE Labs