linux-kernel - Re: [PATCH 1/2] mm/khugepaged: set THP as uptodate earlier for shmem

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Message-ID: <Y+usdhfguWr/aD5x@casper.infradead.org>
Date:   Tue, 14 Feb 2023 15:44:54 +0000
From:   Matthew Wilcox <willy@...radead.org>
To:     David Stevens <stevensd@...omium.org>
Cc:     linux-mm@...ck.org, Peter Xu <peterx@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Yang Shi <shy828301@...il.com>,
        David Hildenbrand <david@...hat.com>,
        Hugh Dickins <hughd@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] mm/khugepaged: set THP as uptodate earlier for shmem

On Tue, Feb 14, 2023 at 04:57:09PM +0900, David Stevens wrote:
>  	/*
> -	 * At this point the hpage is locked and not up-to-date.
> -	 * It's safe to insert it into the page cache, because nobody would
> -	 * be able to map it or use it in another way until we unlock it.
> +	 * Mark hpage as up-to-date before inserting it into the page cache to
> +	 * prevent it from being mistaken for an fallocated but unwritten page.
> +	 * Inserting the unfinished hpage into the page cache is safe because
> +	 * it is locked, so nobody can map it or use it in another way until we
> +	 * unlock it.

No, that's not true.  The data has to be there before we mark it
uptodate.  See filemap_get_pages() for example, used as part of
read().  We don't lock the page unless we need to bring it uptodate
ourselves.