Re: [PATCH v5 3/3] squashfs: implement readahead

["Fabio M. De Francesco" <fmdefrancesco@gmail.com>]

On lunedì 6 giugno 2022 17:03:05 CEST 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.
> - 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>
> Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>
> Reported-by: Andrew Morton <akpm@linux-foundation.org>
> ---
> v4->v5:
> - Handle short file cases reported by Marek and Matthew.
> - Fix checkpatch error reported by Andrew.
> 
> v4: https://lore.kernel.org/lkml/20220601103922.1338320-4-hsinyi@chromium.org/
> 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 | 124 ++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 123 insertions(+), 1 deletion(-)
> 
> diff --git a/fs/squashfs/file.c b/fs/squashfs/file.c
> index a8e495d8eb86..fbd096cd15f4 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 +496,128 @@ 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))
> +			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;
> +
> +		if (nr_pages < max_pages) {
> +			struct squashfs_cache_entry *buffer;
> +			unsigned int block_mask = max_pages - 1;
> +			int offset = pages[0]->index - (pages[0]-
>index & ~block_mask);
> +
> +			buffer = squashfs_get_datablock(inode->i_sb, 
block,
> +							
bsize);
> +			if (buffer->error) {
> +				squashfs_cache_put(buffer);
> +				goto skip_pages;
> +			}
> +
> +			expected -= offset * PAGE_SIZE;
> +			for (i = 0; i < nr_pages && expected > 0; i+
+,
> +						expected -= 
PAGE_SIZE, offset++) {
> +				int avail = min_t(int, expected, 
PAGE_SIZE);
> +
> +				squashfs_fill_page(pages[i], 
buffer,
> +						offset * 
PAGE_SIZE, avail);
> +				unlock_page(pages[i]);
> +			}
> +
> +			squashfs_cache_put(buffer);
> +			continue;
> +		}
> +
> +		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);
> +			}

Hi Hsin-Yi,

kmap_atomic() shouldn't be used in new code, unless there are special 
reasons that I am not able to spot here.

Why not use kmap_local_page(), preferably via memzero_page?

Thanks,

Fabio

> +
> +			for (i = 0; i < nr_pages; i++) {
> +				flush_dcache_page(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
>  };
> -- 
> 2.36.1.255.ge46751e96f-goog
> 
> 
>