Re: squashfs performance regression and readahea

[Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>]

On Mon, May 9, 2022 at 12:45 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> On Sun, May 08, 2022 at 02:46:40PM +0000, Song, Xiongwei wrote:
> > I am facing a performance regression on squashfs.  There are many squashfs
> > partitions on our board. I am doing the operations below on 90 squashfs
> > partitions:
>
> Hi Xiongwei.  Thanks for the report.  Hsin-Ye and I have been working on
> this problem off-list.  Hsin-Ye, could you send the latest version?

Hi Matthew,

This is the patch of the latest version. Since it's just being
reviewed halfway done, I posted the patch here, or should I send it to
the list directly?

Thanks

>From 03558dcaab93ed3c32498eb70c7f2b1382b218d6 Mon Sep 17 00:00:00 2001
From: Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>
Date: Sun, 10 Oct 2021 21:22:25 +0800
Subject: [PATCH] squashfs: implement readahead

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.
- current batch of pages isn't in the same datablock or not enough in a
  datablock.
Otherwise pages will be marked as uptodate. The unhandled pages will be
updated by readpage later.

Signed-off-by: Hsin-Yi Wang <hsinyi@xxxxxxxxxxxx>
---
 fs/squashfs/file.c | 72 +++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 71 insertions(+), 1 deletion(-)

diff --git a/fs/squashfs/file.c b/fs/squashfs/file.c
index 89d492916dea..20ec48cf97c5 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
@@ -494,7 +495,76 @@ static int squashfs_readpage(struct file *file,
struct page *page)
  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;
+ size_t shift = msblk->block_log - PAGE_SHIFT;
+ loff_t req_end = readahead_pos(ractl) + readahead_length(ractl);
+ 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;
+ u64 block = 0;
+ int bsize, res, i, index;
+ int file_end = i_size_read(inode) >> msblk->block_log;
+ int max_pages = 1UL << shift;
+
+ readahead_expand(ractl, start, (len | mask) + 1);
+
+ if (readahead_pos(ractl) + readahead_length(ractl) < req_end)
+ return;
+
+ nr_pages = readahead_count(ractl);
+ pages = kmalloc_array(nr_pages, sizeof(void *), GFP_KERNEL);
+ if (!pages)
+ return;
+
+ actor = squashfs_page_actor_init_special(pages, nr_pages, 0);
+ if (!actor)
+ goto out;
+
+ for (;;) {
+ nr_pages = __readahead_batch(ractl, pages, max_pages);
+ if (!nr_pages)
+ break;
+
+ if (readahead_pos(ractl) >= i_size_read(inode) ||
+     file_end == 0 || nr_pages < max_pages)
+ goto skip_pages;
+
+ index = pages[0]->index >> shift;
+ bsize = read_blocklist(inode, index, &block);
+ res = squashfs_read_data(inode->i_sb, block, bsize, NULL,
+ actor);
+
+ if (res >=0 && (pages[nr_pages - 1]->index >> shift) == index)
+ 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);
+ 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 = {
- .readpage = squashfs_readpage
+ .readpage = squashfs_readpage,
+ .readahead = squashfs_readahead
 };
-- 
2.31.0