From: Michal Hocko <mhocko@kernel.org>
To: Vlastimil Babka <vbabka@suse.cz>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Linus Torvalds <torvalds@linux-foundation.org>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
linux-api@vger.kernel.org, Peter Zijlstra <peterz@infradead.org>,
Greg KH <gregkh@linuxfoundation.org>,
Jann Horn <jannh@google.com>, Jiri Kosina <jkosina@suse.cz>,
Dominique Martinet <asmadeus@codewreck.org>,
Andy Lutomirski <luto@amacapital.net>,
Dave Chinner <david@fromorbit.com>,
Kevin Easton <kevin@guarana.org>,
Matthew Wilcox <willy@infradead.org>,
Cyril Hrubis <chrubis@suse.cz>, Tejun Heo <tj@kernel.org>,
"Kirill A . Shutemov" <kirill@shutemov.name>,
Daniel Gruss <daniel@gruss.cc>, Jiri Kosina <jikos@kernel.org>,
linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is set for the I/O
Date: Thu, 31 Jan 2019 10:56:44 +0100 [thread overview]
Message-ID: <20190131095644.GR18811@dhcp22.suse.cz> (raw)
In-Reply-To: <20190130124420.1834-3-vbabka@suse.cz>
[Cc fs-devel]
On Wed 30-01-19 13:44:19, Vlastimil Babka wrote:
> From: Jiri Kosina <jkosina@suse.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@linux-foundation.org>
> Cc: Dominique Martinet <asmadeus@codewreck.org>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Dave Chinner <david@fromorbit.com>
> Cc: Kevin Easton <kevin@guarana.org>
> Cc: Matthew Wilcox <willy@infradead.org>
> Cc: Cyril Hrubis <chrubis@suse.cz>
> Cc: Tejun Heo <tj@kernel.org>
> Cc: Kirill A. Shutemov <kirill@shutemov.name>
> Cc: Daniel Gruss <daniel@gruss.cc>
> Signed-off-by: Jiri Kosina <jkosina@suse.cz>
> Signed-off-by: Vlastimil Babka <vbabka@suse.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
next parent reply other threads:[~2019-01-31 9:56 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <nycvar.YFH.7.76.1901051817390.16954@cbobk.fhfr.pm>
[not found] ` <20190130124420.1834-1-vbabka@suse.cz>
[not found] ` <20190130124420.1834-3-vbabka@suse.cz>
2019-01-31 9:56 ` Michal Hocko [this message]
2019-01-31 10:15 ` [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT is set for the I/O Jiri Kosina
2019-01-31 10:23 ` Michal Hocko
2019-01-31 10:30 ` Jiri Kosina
2019-01-31 11:32 ` Michal Hocko
2019-01-31 17:54 ` Linus Torvalds
2019-02-01 5:13 ` Dave Chinner
2019-02-01 7:05 ` Linus Torvalds
2019-02-01 7:21 ` Linus Torvalds
2019-02-01 1:44 ` Dave Chinner
2019-02-12 15:48 ` Jiri Kosina
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