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

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

Message-ID: <8da60934-0670-4f8a-8bde-fa4de320cdbb@redhat.com>
Date: Mon, 26 May 2025 09:40:06 +0200
From: David Hildenbrand <david@...hat.com>
To: Dev Jain <dev.jain@....com>, ziy@...dia.com, willy@...radead.org,
 dhowells@...hat.com, hughd@...gle.com
Cc: 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 26.05.25 08:35, 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()?

The usual sequence for the pagecache is (see filemap_get_entry())

1) xas_load(): Load the entry

2) xas_retry(): Test if we have to retry immediately

3) folio || xa_is_value(folio): check if the entry stores a folio

4) folio_try_get(): try getting a folio reference, might get freed
    concurrently, so a folio_get() is not safe

5) folio != xas_reload(&xas): recheck whether the entry was changed
    concurrently

iter_xarray_get_pages()->iter_xarray_populate_pages() works on whatever 
xarray was provided to iov_iter_xarray().

erofs/netfs/orangefs seem to pass the pagecache ... so I would also 
assume that we have to use the same sequence as above.

Willy and me had a look ad that code in b57f4f4f186d ("iov_iter: convert 
iter_xarray_populate_pages() to use folios").

But looking at it now, I think that code is incorrect. At least the 
folio_get() and reload-before-folio-get is weird.

-- 
Cheers,

David / dhildenb