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Date:   Thu, 31 Jan 2019 11:23:48 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Jiri Kosina <jikos@kernel.org>
Cc:     Vlastimil Babka <vbabka@suse.cz>,
        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>,
        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>, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT
 is set for the I/O
Message-ID: <20190131102348.GT18811@dhcp22.suse.cz>
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On Thu 31-01-19 11:15:28, Jiri Kosina wrote:
> On Thu, 31 Jan 2019, Michal Hocko wrote:
> 
> > > 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?
> 
> page_cache_sync_readahead() only submits the read ahead request(s), it 
> doesn't wait for it to finish.

OK, I guess my question was not precise. What does prevent taking fs
locks down the path?
-- 
Michal Hocko
SUSE Labs