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SubjectRe: [PATCH 2/2] squashfs: implement readahead
On Mon, May 16, 2022 at 8:55 PM Matthew Wilcox <willy@infradead.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@infradead.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.

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