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

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Message-ID: <20161024113625.GC2849@node.shutemov.name>
Date:   Mon, 24 Oct 2016 14:36:25 +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 03:18:02PM +0200, Jan Kara wrote:
> On Thu 13-10-16 15:08:44, Kirill A. Shutemov wrote:
> > 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.
> 
> OK, understand that. But why do we need a coherent view? For which purposes
> exactly do we care that it is not just a bunch of 4k pages that happen to
> be physically contiguous and thus can be mapped in one PMD?

My understanding is that things like PageDirty() should be handled on the
same granularity as PAGECACHE_TAG_DIRTY, otherwise things can go horribly
wrong...

-- 
 Kirill A. Shutemov