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

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Message-ID: <20161107150103.GA17451@infradead.org>
Date:   Mon, 7 Nov 2016 07:01:03 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Christoph Hellwig <hch@...radead.org>, Jan Kara <jack@...e.cz>,
        "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 Mon, Nov 07, 2016 at 02:13:05PM +0300, Kirill A. Shutemov wrote:
> It looks like a huge limitation to me.

The DAX PMD fault code can live just fine with it.  And without it
performance would suck anyway.

> I'm looking onto iomap_write_actor(): we still calculate 'offset' and
> 'bytes' based on PAGE_SIZE before we even get the page.
> This way we limit outself to PAGE_SIZE per-iteration.

Of course it does, given that it does not support huge pages _yet_.
But the important part is that this is now isolate to the highlevel
code, and the fs can get iomap_begin calls for a huge page (or in fact
much larger sizes than that).