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Date: Tue, 26 Mar 2024 12:21:40 +0000
From: Matthew Wilcox <willy@infradead.org>
To: Zhaoyang Huang <huangzhaoyang@gmail.com>
Cc: =?utf-8?B?6buE5pyd6ZizIChaaGFveWFuZyBIdWFuZyk=?= <zhaoyang.huang@unisoc.com>,
	Andrew Morton <akpm@linux-foundation.org>,
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	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	=?utf-8?B?5bq357qq5ruoIChTdGV2ZSBLYW5nKQ==?= <Steve.Kang@unisoc.com>
Subject: Re: summarize all information again at bottom//reply: reply: [PATCH]
 mm: fix a race scenario in folio_isolate_lru
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On Tue, Mar 26, 2024 at 05:06:55PM +0800, Zhaoyang Huang wrote:
> 1. Thread_readahead remove the folio from page cache and drop 2 refcnt
> by readahead_folio & filemap_remove_folio(get rid of the folios which
> failed to launch IO during readahead)
>     refcnt == 0, PG_lru == true, PG_lock == true
>     read_pages
> ...
>         folio = readahead_folio
>         <one refcnt dropped here>
> ********For the folio which can not launch IO, we should NOT drop
> refcnt here??? replaced by __readahead_folio???**********
>         folio_get
>         filemap_remove_folio(folio)
>         folio_unlock
>         <one refcnt dropped here>
>         folio_put

Ignoring any other thread, you're basically saying that there's a
refcount imbalance here.  Which means we'd hit an assert (that folio
refcount went below zero) in the normal case where another thread wasn't
simultaneously trying to do anything.