Re: [PATCH v4 3/3] squashfs: implement readahead

[Matthew Wilcox <willy@infradead.org>]

On Fri, Jun 03, 2022 at 10:55:01PM +0800, Hsin-Yi Wang wrote:
> On Fri, Jun 3, 2022 at 10:10 PM Marek Szyprowski
> <m.szyprowski@samsung.com> wrote:
> >
> > Hi Matthew,
> >
> > On 03.06.2022 14:59, Matthew Wilcox wrote:
> > > On Fri, Jun 03, 2022 at 02:54:21PM +0200, Marek Szyprowski wrote:
> > >> On 01.06.2022 12:39, Hsin-Yi Wang wrote:
> > >>> Implement readahead callback for squashfs. It will read datablocks
> > >>> which cover pages in readahead request. For a few cases it will
> > >>> not mark page as uptodate, including:
> > >>> - file end is 0.
> > >>> - zero filled blocks.
> > >>> - current batch of pages isn't in the same datablock or not enough in a
> > >>>     datablock.
> > >>> - decompressor error.
> > >>> Otherwise pages will be marked as uptodate. The unhandled pages will be
> > >>> updated by readpage later.
> > >>>
> > >>> Suggested-by: Matthew Wilcox <willy@infradead.org>
> > >>> Signed-off-by: Hsin-Yi Wang <hsinyi@chromium.org>
> > >>> Reported-by: Matthew Wilcox <willy@infradead.org>
> > >>> Reported-by: Phillip Lougher <phillip@squashfs.org.uk>
> > >>> Reported-by: Xiongwei Song <Xiongwei.Song@windriver.com>
> > >>> ---
> > >> This patch landed recently in linux-next as commit 95f7a26191de
> > >> ("squashfs: implement readahead"). I've noticed that it causes serious
> > >> issues on my test systems (various ARM 32bit and 64bit based boards).
> > >> The easiest way to observe is udev timeout 'waiting for /dev to be fully
> > >> populated' and prolonged booting time. I'm using squashfs for deploying
> > >> kernel modules via initrd. Reverting aeefca9dfae7 & 95f7a26191deon on
> > >> top of the next-20220603 fixes the issue.
> > > How large are these files?  Just a few kilobytes?
> >
> > Yes, they are small, most of them are smaller than 16KB, some about
> > 128KB and a few about 256KB. I've sent a detailed list in private mail.
> >
> 
> Hi Marek,
> 
> Are there any obvious squashfs errors in dmesg? Did you enable
> CONFIG_SQUASHFS_FILE_DIRECT or CONFIG_SQUASHFS_FILE_CACHE?

I don't think it's an error problem.  I think it's a short file problem.

As I understand the current code (and apologies for not keeping up
to date with how the patch is progressing), if the file is less than
msblk->block_size bytes, we'll leave all the pages as !uptodate, leaving
them to be brough uptodate by squashfs_read_folio().  So Marek is hitting
the worst case scenario where we re-read the entire block for each page
in it.  I think we have to handle this tail case in ->readahead().