Re: [PATCH 0/5] fs/buffer: strack reduction on async read

Matthew Wilcox
2024-12-19T03:51:42Z

On Wed, Dec 18, 2024 at 06:27:36PM -0800, Luis Chamberlain wrote:
> On Wed, Dec 18, 2024 at 08:05:29PM +0000, Matthew Wilcox wrote:
> > On Tue, Dec 17, 2024 at 06:26:21PM -0800, Luis Chamberlain wrote:
> > > This splits up a minor enhancement from the bs > ps device support
> > > series into its own series for better review / focus / testing.
> > > This series just addresses the reducing the array size used and cleaning
> > > up the async read to be easier to read and maintain.
> >
> > How about this approach instead -- get rid of the batch entirely?
>
> Less is more! I wish it worked, but we end up with a null pointer on
> ext4/032 (and indeed this is the test that helped me find most bugs in
> what I was working on):

Yeah, I did no testing; just wanted to give people a different approach
to consider.

> [ 106.034851] BUG: kernel NULL pointer dereference, address: 0000000000000000
> [ 106.046300] RIP: 0010:end_buffer_async_read_io+0x11/0x90
> [ 106.047819] Code: f2 ff 0f 1f 80 00 00 00 00 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 90 0f 1f 44 00 00 53 48 8b 47 10 48 89 fb 48 8b 40 18 <48> 8b 00 f6 40 0d 40 74 0d 0f b7 00 66 25 00 f0 66 3d 00 80 74 09

That decodes as:

5: 53 push %rbx
6: 48 8b 47 10 mov 0x10(%rdi),%rax
a: 48 89 fb mov %rdi,%rbx
d: 48 8b 40 18 mov 0x18(%rax),%rax
11:* 48 8b 00 mov (%rax),%rax <-- trapping instruction
14: f6 40 0d 40 testb $0x40,0xd(%rax)

6: bh->b_folio
d: b_folio->mapping
11: mapping->host

So folio->mapping is NULL.

Ah, I see the problem. end_buffer_async_read() uses the buffer_async_read
test to decide if all buffers on the page are uptodate or not. So both
having no batch (ie this patch) and having a batch which is smaller than
the number of buffers in the folio can lead to folio_end_read() being
called prematurely (ie we'll unlock the folio before finishing reading
every buffer in the folio).

Once the folio is unlocked, it can be truncated. That's a second-order
problem, but it's the one your test happened to hit.

This should fix the problem; we always have at least one BH held in
the submission path with the async_read flag set, so
end_buffer_async_read() will not end it prematurely.

By the way, do you have CONFIG_VM_DEBUG enabled in your testing?

VM_BUG_ON_FOLIO(!folio_test_locked(folio), folio);
in folio_end_read() should have tripped before hitting the race with
truncate.

diff --git a/fs/buffer.c b/fs/buffer.c
index cc8452f60251..fd2633e4a5d2 100644
--- a/fs/buffer.c
+++ b/fs/buffer.c
@@ -2361,9 +2361,9 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
{
struct inode *inode = folio->mapping->host;
sector_t iblock, lblock;
- struct buffer_head *bh, *head, *arr[MAX_BUF_PER_PAGE];
+ struct buffer_head *bh, *head, *prev = NULL;
size_t blocksize;
- int nr, i;
+ int i;
int fully_mapped = 1;
bool page_error = false;
loff_t limit = i_size_read(inode);
@@ -2380,7 +2380,6 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
iblock = div_u64(folio_pos(folio), blocksize);
lblock = div_u64(limit + blocksize - 1, blocksize);
bh = head;
- nr = 0;
i = 0;

do {
@@ -2411,40 +2410,33 @@ int block_read_full_folio(struct folio *folio, get_block_t *get_block)
if (buffer_uptodate(bh))
continue;
}
- arr[nr++] = bh;
+
+ lock_buffer(bh);
+ if (buffer_uptodate(bh)) {
+ unlock_buffer(bh);
+ continue;
+ }
+
+ mark_buffer_async_read(bh);
+ if (prev)
+ submit_bh(REQ_OP_READ, prev);
+ prev = bh;
} while (i++, iblock++, (bh = bh->b_this_page) != head);

if (fully_mapped)
folio_set_mappedtodisk(folio);

- if (!nr) {
- /*
- * All buffers are uptodate or get_block() returned an
- * error when trying to map them - we can finish the read.
- */
- folio_end_read(folio, !page_error);
- return 0;
- }
-
- /* Stage two: lock the buffers */
- for (i = 0; i < nr; i++) {
- bh = arr[i];
- lock_buffer(bh);
- mark_buffer_async_read(bh);
- }
-
/*
- * Stage 3: start the IO. Check for uptodateness
- * inside the buffer lock in case another process reading
- * the underlying blockdev brought it uptodate (the sct fix).
+ * All buffers are uptodate or get_block() returned an error
+ * when trying to map them - we must finish the read because
+ * end_buffer_async_read() will never be called on any buffer
+ * in this folio.
*/
- for (i = 0; i < nr; i++) {
- bh = arr[i];
- if (buffer_uptodate(bh))
- end_buffer_async_read(bh, 1);
- else
- submit_bh(REQ_OP_READ, bh);
- }
+ if (prev)
+ submit_bh(REQ_OP_READ, prev);
+ else
+ folio_end_read(folio, !page_error);
+
return 0;
}
EXPORT_SYMBOL(block_read_full_folio);