Re: [PATCH v4 3/3] squashfs: implement readahead
From: Hsin-Yi Wang
Date: Mon Jun 06 2022 - 11:09:19 EST
On Mon, Jun 6, 2022 at 7:09 PM Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx> wrote:
>
> 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.
>
Hi all,
The correct version is sent as v5:
https://lore.kernel.org/lkml/20220606150305.1883410-1-hsinyi@xxxxxxxxxxxx/T/#t
Note that this is based on next-20220513, which doesn't have v4 applied.
I also squashed a fix to a checkpatch error in this version.
Thanks
> > 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
> > >