linux-kernel - Re: [PATCH v2] ext4: check folio uptodate state in ext4_page

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aTCtITpW9yLNm2hz@casper.infradead.org>
Date: Wed, 3 Dec 2025 21:35:29 +0000
From: Matthew Wilcox <willy@...radead.org>
To: Theodore Tso <tytso@....edu>
Cc: Deepanshu Kartikey <kartikey406@...il.com>,
	Zhang Yi <yi.zhang@...weicloud.com>, linux-ext4@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	syzbot+b0a0670332b6b3230a0a@...kaller.appspotmail.com,
	adilger.kernel@...ger.ca, djwong@...nel.org
Subject: Re: [PATCH v2] ext4: check folio uptodate state in
 ext4_page_mkwrite()

You snipped out all the context when adding me to the cc, and I'm on
holiday until after Plumbers, so I'm disinclined to go looking for
context.

On Wed, Dec 03, 2025 at 10:46:57AM -0500, Theodore Tso wrote:
> My main concern with your patch is folio_lock() is *incredibly*
> heavyweight and is going to be a real scalability concern if we need
> to take it every single time we need to make a page writeable.
> 
> So could we perhaps do something like this?  So the first question is
> do we need to take the lock at all?  I'm not sure we need to worry
> about the case where the page is not uptodate because we're racing
> with the page being brought into memory; if we that could happen under
> normal circumstances we would be triggering the warning even without
> these situations such as a delayed allocaiton write failing due to a
> corrupted file system image.   So can we just do this?
> 
> 	if (!folio_test_uptodate(folio)) {
> 		ret = VM_FAULT_SIGBUS;
> 		goto out;
> 	}
> 
> If it is legitmate that ext4_page_mkwrite() could be called while the
> page is still being read in (and again, I don't think it is), then we
> could do something like this:
> 
> 	if (!folio_test_uptodate(folio)) {
> 		folio_lock(folio);
> 		if (!folio_test_uptodate(folio)) {
> 			folio_unlock(folio);
> 			ret = VM_FAULT_SIGBUS;
> 			goto out;
> 		}
> 		folio_unlock(folio);
> 	}
> 
> Matthew, as the page cache maintainer, do we actually need this extra
> rigamarole.  Or can we just skip taking the lock before checking to
> see if the folio is uptodate in ext4_page_mkwrite()?
> 
> 							- Ted