linux-kernel - Re: [PATCHv3 29/41] ext4: make ext4_mpage

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Message-ID: <20160916121740.GA18021@node.shutemov.name>
Date:   Fri, 16 Sep 2016 15:17:40 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Andreas Dilger <adilger@...ger.ca>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Theodore Ts'o <tytso@....edu>, 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 <linux-ext4@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux MM <linux-mm@...ck.org>, linux-block@...r.kernel.org
Subject: Re: [PATCHv3 29/41] ext4: make ext4_mpage_readpages() hugepage-aware

On Thu, Sep 15, 2016 at 06:27:10AM -0600, Andreas Dilger wrote:
> On Sep 15, 2016, at 5:55 AM, Kirill A. Shutemov <kirill.shutemov@...ux.intel.com> wrote:
> > 
> > This patch modifies ext4_mpage_readpages() to deal with huge pages.
> > 
> > We read out 2M at once, so we have to alloc (HPAGE_PMD_NR *
> > blocks_per_page) sector_t for that. I'm not entirely happy with kmalloc
> > in this codepath, but don't see any other option.
> 
> If you're reading 2MB from disk (possibly from disjoint blocks with seeks
> in between) I don't think that the kmalloc() is going to be the limiting
> performance factor.  If you are concerned about the size of the kmalloc()
> causing failures when pages are fragmented (it can be 16KB for 1KB blocks
> with 4KB pages), then using ext4_kvmalloc() to fall back to vmalloc() in
> case kmalloc() fails.  It shouldn't fail often for 16KB allocations,
> but it could in theory.

Good point. Will use ext4_kvmalloc().

> I also notice that ext4_kvmalloc() should probably use unlikely() for
> the failure case, so that the uncommon vmalloc() fallback is out-of-line
> in this more important codepath.  The only other callers are during mount,
> so a branch misprediction is not critical.

I agree. But it's out-of-scope for the patchset.

-- 
 Kirill A. Shutemov