linux-ext4 - Re: [PATCHv3 15/41] filemap: handle huge pages in do_generic_file

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Message-ID: <20161102143612.GA4790@infradead.org>
Date:   Wed, 2 Nov 2016 07:36:12 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     Jan Kara <jack@...e.cz>
Cc:     "Kirill A. Shutemov" <kirill@...temov.name>,
        "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 15/41] filemap: handle huge pages in
 do_generic_file_read()

On Tue, Nov 01, 2016 at 05:39:40PM +0100, Jan Kara wrote:
> I'd also note that having PMD-sized pages has some obvious disadvantages as
> well:
> 
> 1) I'm not sure buffer head handling code will quite scale to 512 or even
> 2048 buffer_heads on a linked list referenced from a page. It may work but
> I suspect the performance will suck. 

buffer_head handling always sucks.  For the iomap based bufferd write
path I plan to support a buffer_head-less mode for the block size ==
PAGE_SIZE case in 4.11 latest, but if I get enough other things of my
plate in time even for 4.10.  I think that's the right way to go for
THP, especially if we require the fs to allocate the whole huge page
as a single extent, similar to the DAX PMD mapping case.

> 2) PMD-sized pages result in increased space & memory usage.

How so?

> 3) In ext4 we have to estimate how much metadata we may need to modify when
> allocating blocks underlying a page in the worst case (you don't seem to
> update this estimate in your patch set). With 2048 blocks underlying a page,
> each possibly in a different block group, it is a lot of metadata forcing
> us to reserve a large transaction (not sure if you'll be able to even
> reserve such large transaction with the default journal size), which again
> makes things slower.

As said above I think we should only use huge page mappings if there is
a single underlying extent, same as in DAX to keep the complexity down.

> 4) As you have noted some places like write_begin() still depend on 4k
> pages which creates a strange mix of places that use subpages and that use
> head pages.

Just use the iomap bufferd I/O code and all these issues will go away.
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