linux-kernel - Re: [PATCH 2/2] squashfs: implement readahead

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Date:   Mon, 16 May 2022 20:57:26 +0800
From:   Hsin-Yi Wang <hsinyi@...omium.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Phillip Lougher <phillip@...ashfs.org.uk>,
        Xiongwei Song <Xiongwei.Song@...driver.com>,
        Zheng Liang <zhengliang6@...wei.com>,
        Zhang Yi <yi.zhang@...wei.com>, Hou Tao <houtao1@...wei.com>,
        Miao Xie <miaoxie@...wei.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "linux-mm @ kvack . org" <linux-mm@...ck.org>,
        "squashfs-devel @ lists . sourceforge . net" 
        <squashfs-devel@...ts.sourceforge.net>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] squashfs: implement readahead

On Mon, May 16, 2022 at 8:55 PM Matthew Wilcox <willy@...radead.org> wrote:
>
> On Mon, May 16, 2022 at 08:47:52PM +0800, Hsin-Yi Wang wrote:
> > On Mon, May 16, 2022 at 8:36 PM Matthew Wilcox <willy@...radead.org> wrote:
> > >
> > > On Mon, May 16, 2022 at 07:04:08PM +0800, Hsin-Yi Wang wrote:
> > > > > +       loff_t req_end = readahead_pos(ractl) + readahead_length(ractl);
> > > > > +       loff_t start = readahead_pos(ractl) &~ mask;
> > > > > +       size_t len = readahead_length(ractl) + readahead_pos(ractl) - start;
> > > > > +       struct squashfs_page_actor *actor;
> > > > > +       unsigned int nr_pages = 0;
> > > > > +       struct page **pages;
> > > > > +       u64 block = 0;
> > > > > +       int bsize, res, i, index;
> > > > > +       int file_end = i_size_read(inode) >> msblk->block_log;
> > > > > +       unsigned int max_pages = 1UL << shift;
> > > > > +
> > > > > +       readahead_expand(ractl, start, (len | mask) + 1);
> > > > > +
> > > > > +       if (readahead_pos(ractl) + readahead_length(ractl) < req_end ||
> > > > > +           file_end == 0)
> > > > > +               return;
> > >
> > > What's the first half of this condition supposed to be checking for?
> > > It seems to be checking whether readahead_expand() shrunk the range
> > > covered by the ractl, but readahead_expand() never does that, so I'm
> > > confused why you're checking for it.
> >
> > hi Matthew,
> >
> > This is to check if readahead_expand() expands as much as it's requested.
> > I didn't encounter the mismatch so far in my testing. If this check is
> > not necessary, it can be removed.
>
> Then I think req_end is miscalculated?  It should surely be:
>
>         req_end = start + (len | mask) + 1;
>
> But I'm not sure that we should be failing under such circumstances.
> For example, we may have been asked to read 1.5MB, attempt to round up
> to 2MB, and fail.  But we can still submit a readahead for the first 1MB,
> before leaving the second 512kB for readpage to handle.
>
> So maybe we should just remove this check entirely.

I'll remove this in the next version.