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

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Date:   Thu, 31 Jan 2019 12:32:23 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Jiri Kosina <jikos@...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 31-01-19 11:30:24, 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?
> 
> 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.

No, it can reschedule for sure but the man page says: 
: If this flag is specified, the preadv2() system call will return
: instantly if it would have to read data from the backing storage or wait
: for a lock.

I assume that the lock is meant to be a filesystem lock here.
-- 
Michal Hocko
SUSE Labs