linux-kernel - Re: [PATCH hotfix 1/2] mm/thp: fix deferred split queue not partially

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Message-ID: <5d28df34-f073-dec5-730e-a3073f14d849@google.com>
Date: Sat, 26 Oct 2024 22:08:13 -0700 (PDT)
From: Hugh Dickins <hughd@...gle.com>
To: Yang Shi <shy828301@...il.com>
cc: Zi Yan <ziy@...dia.com>, Hugh Dickins <hughd@...gle.com>, 
    Andrew Morton <akpm@...ux-foundation.org>, 
    Usama Arif <usamaarif642@...il.com>, Wei Yang <richard.weiyang@...il.com>, 
    "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, 
    Matthew Wilcox <willy@...radead.org>, David Hildenbrand <david@...hat.com>, 
    Johannes Weiner <hannes@...xchg.org>, 
    Baolin Wang <baolin.wang@...ux.alibaba.com>, 
    Barry Song <baohua@...nel.org>, Kefeng Wang <wangkefeng.wang@...wei.com>, 
    Ryan Roberts <ryan.roberts@....com>, Nhat Pham <nphamcs@...il.com>, 
    Chris Li <chrisl@...nel.org>, linux-kernel@...r.kernel.org, 
    linux-mm@...ck.org
Subject: Re: [PATCH hotfix 1/2] mm/thp: fix deferred split queue not
 partially_mapped

On Fri, 25 Oct 2024, Yang Shi wrote:
> 
> The other subtle thing is folio->_deferred_list is reused when the
> folio is moved to the local on-stack list. And some

Yes.

> list_empty(deferred_list) checks return true even though the folio is
> actually on the local on-stack list. Some code may depend on or

The code definitely depends on that behaviour: that's how folios get
unqueued when refcount reaches 0, whether they are on the public list
or on the local list at that time.

> inadvertently depend on this behavior. Using folio_batch may break
> some assumptions, but depending on this subtle behavior is definitely
> not reliable IMHO.