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Date:   Thu, 31 Jan 2019 11:15:28 +0100 (CET)
From:   Jiri Kosina <jikos@kernel.org>
To:     Michal Hocko <mhocko@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
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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.

-- 
Jiri Kosina
SUSE Labs