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

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Date:   Thu, 31 Jan 2019 11:30:24 +0100 (CET)
From:   Jiri Kosina <jikos@...nel.org>
To:     Michal Hocko <mhocko@...nel.org>
cc:     Vlastimil Babka <vbabka@...e.cz>,
        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>,
        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>, linux-fsdevel@...r.kernel.org
Subject: Re: [PATCH 2/3] mm/filemap: initiate readahead even if IOCB_NOWAIT
 is set for the I/O

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?

Well, RWF_NOWAIT doesn't mean the kernel can't reschedule while executing 
preadv2(), right? It just means it will not wait for the arrival of the 
whole data blob into pagecache in case it's not there.

-- 
Jiri Kosina
SUSE Labs