linux-kernel - Re: [QUESTION] xas_reload() in iter_xarray_populate

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 for Android: free password hash cracker in your pocket

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

Message-ID: <aFFx64M_iFgeIh5j@casper.infradead.org>
Date: Tue, 17 Jun 2025 14:47:23 +0100
From: Matthew Wilcox <willy@...radead.org>
To: Dev Jain <dev.jain@....com>
Cc: david@...hat.com, ziy@...dia.com, dhowells@...hat.com, hughd@...gle.com,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org,
	ryan.roberts@....com, aneesh.kumar@...nel.org
Subject: Re: [QUESTION] xas_reload() in iter_xarray_populate_pages()

On Tue, Jun 17, 2025 at 10:40:51AM +0530, Dev Jain wrote:
> 
> On 26/05/25 12:05 pm, Dev Jain wrote:
> > Hello all,
> > 
> > After doing an xas_load() and xas_retry(), we take neither a reference nor a lock
> > on the folio, and we do an xas_reload(). Is this just to reduce the time window
> > for a race?
> > 
> > If the above is true, then, there is a negligible window between xas_load() and
> > xas_reload(), because only xas_retry() exists between them, so why to even reload()?
> > 
> > Thanks,
> > Dev
> 
> I do not completely remember our discussion in THP Cabal; I recall David Howells maybe
> saying that the folios are already locked, so it is safe to do xas_load and then do
> a folio_get()? Even if we remove the redundant xas_reload(), I still don't understand
> why we won't need xas_reload() at least after folio_get()?

Because you need xas_reload() in order to solve this race:

A: load folio
B: remove folio
B: free folio
C: alloc folio
A: tryget folio
A: reload folio

If A already has a refcount on folio, folio cannot be freed, and so A
cannot get a refcount to C's folio.

The other mutexes are irrelevant here; this is purely a folio refcount
problem/solution.