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

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hi,

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.

Let me know how I can help debugging this issue. There is no hurry 
though, because the next week I will be on holidays.

> v3->v4: Fix a few variable type and their locations.
> v3: https://lore.kernel.org/lkml/20220523065909.883444-4-hsinyi@chromium.org/
> v2: https://lore.kernel.org/lkml/20220517082650.2005840-4-hsinyi@chromium.org/
> v1: https://lore.kernel.org/lkml/20220516105100.1412740-3-hsinyi@chromium.org/
> ---
>   fs/squashfs/file.c | 97 +++++++++++++++++++++++++++++++++++++++++++++-
>   1 file changed, 96 insertions(+), 1 deletion(-)
>
> diff --git a/fs/squashfs/file.c b/fs/squashfs/file.c
> index a8e495d8eb86..df7ad4b3e99c 100644
> --- a/fs/squashfs/file.c
> +++ b/fs/squashfs/file.c
> @@ -39,6 +39,7 @@
>   #include "squashfs_fs_sb.h"
>   #include "squashfs_fs_i.h"
>   #include "squashfs.h"
> +#include "page_actor.h"
>   
>   /*
>    * Locate cache slot in range [offset, index] for specified inode.  If
> @@ -495,7 +496,101 @@ static int squashfs_read_folio(struct file *file, struct folio *folio)
>   	return 0;
>   }
>   
> +static void squashfs_readahead(struct readahead_control *ractl)
> +{
> +	struct inode *inode = ractl->mapping->host;
> +	struct squashfs_sb_info *msblk = inode->i_sb->s_fs_info;
> +	size_t mask = (1UL << msblk->block_log) - 1;
> +	unsigned short shift = msblk->block_log - PAGE_SHIFT;
> +	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;
> +	int i, file_end = i_size_read(inode) >> msblk->block_log;
> +	unsigned int max_pages = 1UL << shift;
> +
> +	readahead_expand(ractl, start, (len | mask) + 1);
> +
> +	if (file_end == 0)
> +		return;
> +
> +	pages = kmalloc_array(max_pages, sizeof(void *), GFP_KERNEL);
> +	if (!pages)
> +		return;
> +
> +	actor = squashfs_page_actor_init_special(pages, max_pages, 0);
> +	if (!actor)
> +		goto out;
> +
> +	for (;;) {
> +		pgoff_t index;
> +		int res, bsize;
> +		u64 block = 0;
> +		unsigned int expected;
> +
> +		nr_pages = __readahead_batch(ractl, pages, max_pages);
> +		if (!nr_pages)
> +			break;
> +
> +		if (readahead_pos(ractl) >= i_size_read(inode) ||
> +		    nr_pages < max_pages)
> +			goto skip_pages;
> +
> +		index = pages[0]->index >> shift;
> +		if ((pages[nr_pages - 1]->index >> shift) != index)
> +			goto skip_pages;
> +
> +		expected = index == file_end ?
> +			   (i_size_read(inode) & (msblk->block_size - 1)) :
> +			    msblk->block_size;
> +
> +		bsize = read_blocklist(inode, index, &block);
> +		if (bsize == 0)
> +			goto skip_pages;
> +
> +		res = squashfs_read_data(inode->i_sb, block, bsize, NULL,
> +					 actor);
> +
> +		if (res == expected) {
> +			int bytes;
> +
> +			/* Last page may have trailing bytes not filled */
> +			bytes = res % PAGE_SIZE;
> +			if (bytes) {
> +				void *pageaddr;
> +
> +				pageaddr = kmap_atomic(pages[nr_pages - 1]);
> +				memset(pageaddr + bytes, 0, PAGE_SIZE - bytes);
> +				kunmap_atomic(pageaddr);
> +			}
> +
> +			for (i = 0; i < nr_pages; i++)
> +				SetPageUptodate(pages[i]);
> +		}
> +
> +		for (i = 0; i < nr_pages; i++) {
> +			unlock_page(pages[i]);
> +			put_page(pages[i]);
> +		}
> +	}
> +
> +	kfree(actor);
> +	kfree(pages);
> +	return;
> +
> +skip_pages:
> +	for (i = 0; i < nr_pages; i++) {
> +		unlock_page(pages[i]);
> +		put_page(pages[i]);
> +	}
> +
> +	kfree(actor);
> +out:
> +	kfree(pages);
> +}
>   
>   const struct address_space_operations squashfs_aops = {
> -	.read_folio = squashfs_read_folio
> +	.read_folio = squashfs_read_folio,
> +	.readahead = squashfs_readahead
>   };

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland