linux-kernel - Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB

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: <20190131095644.GR18811@dhcp22.suse.cz>
Date:   Thu, 31 Jan 2019 10:56:44 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Vlastimil Babka <vbabka@...e.cz>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-api@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
        Greg KH <gregkh@...uxfoundation.org>,
        Jann Horn <jannh@...gle.com>, Jiri Kosina <jkosina@...e.cz>,
        Dominique Martinet <asmadeus@...ewreck.org>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Chinner <david@...morbit.com>,
        Kevin Easton <kevin@...rana.org>,
        Matthew Wilcox <willy@...radead.org>,
        Cyril Hrubis <chrubis@...e.cz>, Tejun Heo <tj@...nel.org>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        Daniel Gruss <daniel@...ss.cc>, Jiri Kosina <jikos@...nel.org>,
        linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT
 is set for the I/O

[Cc fs-devel]

On Wed 30-01-19 13:44:19, Vlastimil Babka wrote:
> From: Jiri Kosina <jkosina@...e.cz>
> 
> preadv2(RWF_NOWAIT) can be used to open a side-channel to pagecache contents, as
> it reveals metadata about residency of pages in pagecache.
> 
> If preadv2(RWF_NOWAIT) returns immediately, it provides a clear "page not
> resident" information, and vice versa.
> 
> Close that sidechannel by always initiating readahead on the cache if we
> encounter a cache miss for preadv2(RWF_NOWAIT); with that in place, probing
> the pagecache residency itself will actually populate the cache, making the
> sidechannel useless.

I guess the current wording doesn't disallow background IO to be
triggered for EAGAIN case. I am not sure whether that breaks clever
applications which try to perform larger IO for those cases though.

> Originally-by: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Dominique Martinet <asmadeus@...ewreck.org>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Dave Chinner <david@...morbit.com>
> Cc: Kevin Easton <kevin@...rana.org>
> Cc: Matthew Wilcox <willy@...radead.org>
> Cc: Cyril Hrubis <chrubis@...e.cz>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Kirill A. Shutemov <kirill@...temov.name>
> Cc: Daniel Gruss <daniel@...ss.cc>
> Signed-off-by: Jiri Kosina <jkosina@...e.cz>
> Signed-off-by: Vlastimil Babka <vbabka@...e.cz>
> ---
>  mm/filemap.c | 2 --
>  1 file changed, 2 deletions(-)
> 
> diff --git a/mm/filemap.c b/mm/filemap.c
> index 9f5e323e883e..7bcdd36e629d 100644
> --- a/mm/filemap.c
> +++ b/mm/filemap.c
> @@ -2075,8 +2075,6 @@ static ssize_t generic_file_buffered_read(struct kiocb *iocb,
>  
>  		page = find_get_page(mapping, index);
>  		if (!page) {
> -			if (iocb->ki_flags & IOCB_NOWAIT)
> -				goto would_block;
>  			page_cache_sync_readahead(mapping,
>  					ra, filp,
>  					index, last_index - index);

Maybe a stupid question but I am not really familiar with this path but
what exactly does prevent a sync read down page_cache_sync_readahead
path?
-- 
Michal Hocko
SUSE Labs