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

[Hsin-Yi Wang]

On Mon, Jun 6, 2022 at 5:55 PM Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx> wrote:
>
> On Mon, Jun 6, 2022 at 11:54 AM Phillip Lougher <phillip@xxxxxxxxxxxxxxx> wrote:
> >
> > On 03/06/2022 16:58, Marek Szyprowski wrote:
> > > Hi Matthew,
> > >
> > > On 03.06.2022 17:29, Matthew Wilcox wrote:
> > >> 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@xxxxxxxxxxx> 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@xxxxxxxxxxxxx>
> > >>>>>>> Signed-off-by: Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>
> > >>>>>>> Reported-by: Matthew Wilcox <willy@xxxxxxxxxxxxx>
> > >>>>>>> Reported-by: Phillip Lougher <phillip@xxxxxxxxxxxxxxx>
> > >>>>>>> Reported-by: Xiongwei Song <Xiongwei.Song@xxxxxxxxxxxxx>
> > >>>>>>> ---
> > >>>>>> 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().
> > >
> > > I'm not sure if this is related to reading of small files. There are
> > > only 50 modules being loaded from squashfs volume. I did a quick test of
> > > reading the files.
> > >
> > > Simple file read with this patch:
> > >
> > > root@target:~# time find /initrd/ -type f | while read f; do cat $f
> > >   >/dev/null; done
> > >
> > > real    0m5.865s
> > > user    0m2.362s
> > > sys     0m3.844s
> > >
> > > Without:
> > >
> > > root@target:~# time find /initrd/ -type f | while read f; do cat $f
> > >   >/dev/null; done
> > >
> > > real    0m6.619s
> > > user    0m2.112s
> > > sys     0m4.827s
> > >
> >
> > It has been a four day holiday in the UK (Queen's Platinum Jubilee),
> > hence the delay in responding.
> >
> > The above read use-case is sequential (only one thread/process),
> > whereas the use-case where the slow-down is observed may be
> > parallel (multiple threads/processes entering Squashfs).
> >
> > The above sequential use-case if the small files are held in
> > fragments, will be exhibiting caching behaviour that will
> > ameliorate the case where the same block is being repeatedly
> > re-read for each page in it.  Because each time
> > Squashfs is re-entered handling only a single page, the
> > decompressed block will be found in the fragment
> > cache, eliminating a block decompression for each page.
> >
> > In a parallel use-case the decompressed fragment block
> > may be being eliminated from the cache (by other reading
> > processes), hence forcing the block to be repeatedly
> > decompressed.
> >
> > Hence the slow-down will be much more noticable with a
> > parallel use-case than a sequential use-case.  It also may
> > be why this slipped through testing, if the test cases
> > are purely sequential in nature.
> >
> > So Matthew's previous comment is still the most likely
> > explanation for the slow-down.
> >
> Thanks for the pointers. To deal with short file cases (nr_pages <
> max_pages), Can we refer to squashfs_fill_page() used in
> squashfs_read_cache(), similar to the case where there are missing
> pages on the block?
>
> Directly calling squashfs_read_data() on short files will lead to crash:
>
> Unable to handle kernel paging request at virtual address:
> [   19.244654]  zlib_inflate+0xba4/0x10c8
> [   19.244658]  zlib_uncompress+0x150/0x1bc
> [   19.244662]  squashfs_decompress+0x6c/0xb4
> [   19.244669]  squashfs_read_data+0x1a8/0x298
> [   19.244673]  squashfs_readahead+0x2cc/0x4cc
>
> I also noticed that the function didn't set flush_dcache_page() with
> SetPageUptodate() previously.
>
> Put these 2 issues together:
>

The patch here is not correct. Please ignore it for now. Sorry for the noice.

> diff --git a/fs/squashfs/file.c b/fs/squashfs/file.c
> index 658fb98af0cd..27519f1f9045 100644
> --- a/fs/squashfs/file.c
> +++ b/fs/squashfs/file.c
> @@ -532,8 +532,7 @@ static void squashfs_readahead(struct
> readahead_control *ractl)
>                 if (!nr_pages)
>                         break;
>
> -               if (readahead_pos(ractl) >= i_size_read(inode) ||
> -                   nr_pages < max_pages)
> +               if (readahead_pos(ractl) >= i_size_read(inode))
>                         goto skip_pages;
>
>                 index = pages[0]->index >> shift;
> @@ -548,6 +547,23 @@ static void squashfs_readahead(struct
> readahead_control *ractl)
>                 if (bsize == 0)
>                         goto skip_pages;
>
> +               if (nr_pages < max_pages) {
> +                       struct squashfs_cache_entry *buffer;
> +
> +                       buffer = squashfs_get_datablock(inode->i_sb, block,
> +                                                       bsize);
> +                       if (!buffer->error) {
> +                               for (i = 0; i < nr_pages && expected > 0; i++,
> +                                                       expected -= PAGE_SIZE) {
> +                                       int avail = min_t(int,
> expected, PAGE_SIZE);
> +
> +                                       squashfs_fill_page(pages[i],
> buffer, i * PAGE_SIZE, avail);
> +                               }
> +                       }
> +                       squashfs_cache_put(buffer);
> +                       goto skip_pages;
> +               }
> +
>                 res = squashfs_read_data(inode->i_sb, block, bsize, NULL,
>                                          actor);
>
> @@ -564,8 +580,10 @@ static void squashfs_readahead(struct
> readahead_control *ractl)
>                                 kunmap_atomic(pageaddr);
>                         }
>
> -                       for (i = 0; i < nr_pages; i++)
> +                       for (i = 0; i < nr_pages; i++) {
> +                               flush_dcache_page(pages[i]);
>                                 SetPageUptodate(pages[i]);
> +                       }
>                 }
>
>
> > Phillip
> >
> > > Best regards
> >