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

[Matthew Wilcox]

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.