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Received: from willy by casper.infradead.org with local (Exim 4.97.1 #2 (Red Hat Linux)) id 1rtvbL-00000000Z3x-2Xvc; Mon, 08 Apr 2024 20:28:23 +0000 Date: Mon, 8 Apr 2024 21:28:23 +0100 From: Matthew Wilcox To: David Hildenbrand Cc: Andrew Morton , linux-mm@kvack.org, "Vishal Moola (Oracle)" Subject: Re: [PATCH 7/7] khugepaged: Use a folio throughout hpage_collapse_scan_file() Message-ID: References: <20240403171838.1445826-1-willy@infradead.org> <20240403171838.1445826-8-willy@infradead.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Rspam-User: X-Stat-Signature: 4rwwemoszdt6bmu936oy6txx8izohf5q X-Rspamd-Server: rspam07 X-Rspamd-Queue-Id: 428F31C0005 X-HE-Tag: 1712608105-205477 X-HE-Meta: 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 7qlA4AhE n/sYXDWmh/8K7KqEz8oUuJ55pvHNr/bZDJ1IRPM+lUPuwtX7LSgmhbP+fPeCD9CChD3hMVp7V/TQqx3tdCX71Gxh5DRq3dImcxqKCcNz6Xf8e8mj/605WnbPcB1RSGoJUeR4tOSJ3zQSrQPcYnJIixWrGb2sBv2F4BqrTSO32kLzPdB/DE+7jJyTLdIZGyWdoxOzU8gnyUMfnrHhSHdep8sgcVUK4VM3Y7VG7p9hlXYpEixuYH57UERqBfCZPS9u8dDxEWWzkptVkOgxlZwtU5tx+BeixmQZ8KN5DG5fg0q92O1wxmz4C5wzV/Q== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On Mon, Apr 08, 2024 at 10:07:01PM +0200, David Hildenbrand wrote: > On 03.04.24 19:18, Matthew Wilcox (Oracle) wrote: > > Replace the use of pages with folios. Saves a few calls to > > compound_head() and removes some uses of obsolete functions. > > > > Signed-off-by: Matthew Wilcox (Oracle) > > --- > > [...] > > > - if (page_count(page) != > > - 1 + page_mapcount(page) + page_has_private(page)) { > > + if (folio_ref_count(folio) != > > + 1 + folio_mapcount(folio) + folio_test_private(folio)) { > > result = SCAN_PAGE_COUNT; > > Just stumbled over that: page_has_private() would have checked PG_private > and PG_private_2. The corresponding replacement would have been > folio_has_private(). folio_test_private() only checks PG_private. > > Are we sure that we no longer have to check PG_private_2 here? pagecache ... > so I have no clue :) Oh man. Vishal just asked me about this in our meeting and I struggled to explain the whole horrid history. The short answer is that this almost certainly fixes a bug. We have a rule (which most filesystem developers don't know about) that attaching private data to the folio should increase its refcount by one. This is handled by folio_attach_private(). But there's no corresponding rule that setting PG_private_2 should also increment the refcount. So checking PG_private_2 is wrong here because a folio with PG_private_2 set and PG_private clear will not have its refcount increased. There are longer versions of this answer, and if you keep asking, I'll give them ;-P