linux-kernel - Re: [PATCH 5/5] mm/madvise: eliminate very confusing manipulation of prev VMA

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Message-ID: <c9a1c792-ef57-49d1-a17d-31e45dd0d8d0@suse.cz>
Date: Fri, 20 Jun 2025 17:02:41 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>,
 Andrew Morton <akpm@...ux-foundation.org>
Cc: David Hildenbrand <david@...hat.com>, Zi Yan <ziy@...dia.com>,
 Baolin Wang <baolin.wang@...ux.alibaba.com>,
 "Liam R . Howlett" <Liam.Howlett@...cle.com>, Nico Pache
 <npache@...hat.com>, Ryan Roberts <ryan.roberts@....com>,
 Dev Jain <dev.jain@....com>, Barry Song <baohua@...nel.org>,
 Jann Horn <jannh@...gle.com>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Lance Yang <ioworker0@...il.com>,
 SeongJae Park <sj@...nel.org>, Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH 5/5] mm/madvise: eliminate very confusing manipulation of
 prev VMA

On 6/19/25 22:26, Lorenzo Stoakes wrote:
> The madvise code has for the longest time had very confusing code around
> the 'prev' VMA pointer passed around various functions which, in all cases
> except madvise_update_vma(), is unused and instead simply updated as soon
> as the function is invoked.
> 
> To compound the confusion, the prev pointer is also used to indicate to the
> caller that the mmap lock has been dropped and that we can therefore not
> safely access the end of the current VMA (which might have been updated by
> madvise_update_vma()).
> 
> Clear up this confusion by not setting prev = vma anywhere except in
> madvise_walk_vmas(), update all references to prev which will always be
> equal to vma after madvise_vma_behavior() is invoked, and adding a flag to
> indicate that the lock has been dropped to make this explicit.
> 
> Additionally, drop a redundant BUG_ON() from madvise_collapse(), which is
> simply reiterating the BUG_ON(mmap_locked) above it (note that BUG_ON() is
> not appropriate here, but we leave existing code as-is).
> 
> We finally adjust the madvise_walk_vmas() logic to be a little clearer -
> delaying the assignment of the end of the range to the start of the new
> range until the last moment and handling the lock being dropped scenario
> immediately.
> 
> Additionally add some explanatory comments.
> 
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@...cle.com>

Reviewed-by: Vlastimil Babka <vbabka@...e.cz>

Much nicer now, thanks!