On Sun, 7 Oct 2007 15:20:19 -0400
Erez Zadok <ezk@xxxxxxxxxxxxx> wrote:
> According to vfs.txt, ->writepage() may return AOP_WRITEPAGE_ACTIVATE back
> to the VFS/VM. Indeed some filesystems such as tmpfs can return
> AOP_WRITEPAGE_ACTIVATE; and stackable file systems (e.g., Unionfs) also
> return AOP_WRITEPAGE_ACTIVATE if the lower f/s returned it.
>
> Anyway, some Ubuntu users of Unionfs reported that msync(2) sometimes
> returns AOP_WRITEPAGE_ACTIVATE (decimal 524288) back to userland.
> Therefore, some user programs fail, esp. if they're written such as this:
>
> err = msync(...);
> if (err != 0)
> // fail
>
> They temporarily fixed the specific program in question (apt-get) to check
>
> if (err < 0)
> // fail
>
> Is this a bug indeed, or are user programs supposed to handle
> AOP_WRITEPAGE_ACTIVATE (I hope not the latter). If it's a kernel bug, what
> should the kernel return: a zero, or an -errno (and which one)?
>
shit. That's a nasty bug. Really userspace should be testing for -1, but
the msync() library function should only ever return 0 or -1.
Does this fix it?
--- a/mm/page-writeback.c~a
+++ a/mm/page-writeback.c
@@ -850,8 +850,10 @@ retry:
ret = (*writepage)(page, wbc, data);
- if (unlikely(ret == AOP_WRITEPAGE_ACTIVATE))
+ if (unlikely(ret == AOP_WRITEPAGE_ACTIVATE)) {
unlock_page(page);
+ ret = 0;
+ }
if (ret || (--(wbc->nr_to_write) <= 0))
done = 1;
if (wbc->nonblocking && bdi_write_congested(bdi)) {
_
-
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