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

Matthew Wilcox
2025-01-31T22:01:54Z

On Fri, Jan 31, 2025 at 08:54:31AM -0800, Luis Chamberlain wrote:
> On Thu, Dec 19, 2024 at 03:51:34AM +0000, Matthew Wilcox wrote:
> > 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).
>
> But:
>
> a) all batched buffers are locked in the old code, we only unlock
> the currently evaluated buffer, the buffers from our pivot are locked
> and should also have the async flag set. That fact that buffers ahead
> should have the async flag set should prevent from calling
> folio_end_read() prematurely as I read the code, no?

I'm sure you know what you mean by "the old code", but I don't.

If you mean "the code in 6.13", here's what it does:

tmp = bh;
do {
if (!buffer_uptodate(tmp))
folio_uptodate = 0;
if (buffer_async_read(tmp)) {
BUG_ON(!buffer_locked(tmp));
goto still_busy;
}
tmp = tmp->b_this_page;
} while (tmp != bh);
folio_end_read(folio, folio_uptodate);

so it's going to cycle around every buffer on the page, and if it finds
none which are marked async_read, it'll call folio_end_read().
That's fine in 6.13 because in stage 2, all buffers which are part of
this folio are marked as async_read.

In your patch, you mark every buffer _in the batch_ as async_read
and then submit the entire batch. So if they all complete before you
mark the next bh as being uptodate, it'll think the read is complete
and call folio_end_read().