linux-ext4 - Re: [PATCHv3 17/41] filemap: handle huge pages in filemap_fdatawait

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Date:   Thu, 13 Oct 2016 15:08:44 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Jan Kara <jack@...e.cz>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Theodore Ts'o <tytso@....edu>,
        Andreas Dilger <adilger.kernel@...ger.ca>,
        Jan Kara <jack@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Hugh Dickins <hughd@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Matthew Wilcox <willy@...radead.org>,
        Ross Zwisler <ross.zwisler@...ux.intel.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        linux-block@...r.kernel.org
Subject: Re: [PATCHv3 17/41] filemap: handle huge pages in
 filemap_fdatawait_range()

On Thu, Oct 13, 2016 at 11:44:41AM +0200, Jan Kara wrote:
> On Thu 15-09-16 14:54:59, Kirill A. Shutemov wrote:
> > We writeback whole huge page a time.
> 
> This is one of the things I don't understand. Firstly I didn't see where
> changes of writeback like this would happen (maybe they come later).
> Secondly I'm not sure why e.g. writeback should behave atomically wrt huge
> pages. Is this because radix-tree multiorder entry tracks dirtiness for us
> at that granularity?

We track dirty/writeback on per-compound pages: meaning we have one
dirty/writeback flag for whole compound page, not on every individual
4k subpage. The same story for radix-tree tags.

> BTW, can you also explain why do we need multiorder entries? What do
> they solve for us?

It helps us having coherent view on tags in radix-tree: no matter which
index we refer from the range huge page covers we will get the same
answer on which tags set.

> I'm sorry for these basic questions but I'd just like to understand how is
> this supposed to work...
> 
> 								Honza
> 
> 
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > ---
> >  mm/filemap.c | 5 +++++
> >  1 file changed, 5 insertions(+)
> > 
> > diff --git a/mm/filemap.c b/mm/filemap.c
> > index 05b42d3e5ed8..53da93156e60 100644
> > --- a/mm/filemap.c
> > +++ b/mm/filemap.c
> > @@ -372,9 +372,14 @@ static int __filemap_fdatawait_range(struct address_space *mapping,
> >  			if (page->index > end)
> >  				continue;
> >  
> > +			page = compound_head(page);
> >  			wait_on_page_writeback(page);
> >  			if (TestClearPageError(page))
> >  				ret = -EIO;
> > +			if (PageTransHuge(page)) {
> > +				index = page->index + HPAGE_PMD_NR;
> > +				i += index - pvec.pages[i]->index - 1;
> > +			}
> >  		}
> >  		pagevec_release(&pvec);
> >  		cond_resched();
> > -- 
> > 2.9.3
> > 
> > 
> -- 
> Jan Kara <jack@...e.com>
> SUSE Labs, CR
> 
> --
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-- 
 Kirill A. Shutemov
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