On 10/7/07, Pekka J Enberg wrote:
> On 10/7/07, Erez Zadok wrote:
> > 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:
>
....
> It's a kernel bug. AOP_WRITEPAGE_ACTIVATE is a hint to the VM to avoid
> writeback of the page in the near future. I wonder if it's enough that we
> change the return value to zero from
> mm/page-writeback.c:write_cache_pages() in case we hit AOP_WRITEPAGE_ACTIVE...

Doesn't appear to be enough. I can't figure out why (since it appears
write_cache_pages bubbles up directly to sys_msync), but with that
patch applied, in my test case[1], msync returns -1 EIO. However,
with the exact same kernel without that patch applied, msync returns
524288 (AOP_WRITEPAGE_ACTIVATE). But as your patch specifically flips
524288 to 0, I can't figure out how it eventually returns -1 EIO.

Ryan

[1] "apt-get check" on a unionfs2 mount backed by tmpfs over cdrom,
standard livecd setup
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Hi Ryan,

On 10/8/07, Ryan Finnie wrote:
> Doesn't appear to be enough. I can't figure out why (since it appears
> write_cache_pages bubbles up directly to sys_msync), but with that
> patch applied, in my test case[1], msync returns -1 EIO. However,
> with the exact same kernel without that patch applied, msync returns
> 524288 (AOP_WRITEPAGE_ACTIVATE). But as your patch specifically flips
> 524288 to 0, I can't figure out how it eventually returns -1 EIO.
>
> [1] "apt-get check" on a unionfs2 mount backed by tmpfs over cdrom,
> standard livecd setup

You have swap device disabled, right? If so, I can't see any reason
why msync(2) on tmpfs would return -EIO. Can you please send a strace
log for your test case?

Pekka
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