Messages in this thread |  | | | Date | Tue, 30 May 2006 12:54:53 +1000 | | From | Nick Piggin <> | | Subject | Re: [rfc][patch] remove racy sync_page? |
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Andrew Morton wrote:
>On Tue, 30 May 2006 10:08:06 +1000 >Nick Piggin <nickpiggin@yahoo.com.au> wrote: > > >>Which is what I want to know. I don't exactly have an interesting >>disk setup. >> > >You don't need one - just a single disk should show up such problems. I >forget which workloads though. Perhaps just a linear read (readahead >queues the I/O but doesn't unplug, subsequent lock_page() sulks). >
I guess so. Is plugging still needed now that the IO layer should get larger requests? Disabling it might result in a small initial request (although even that may be good for pipelining)...
Otherwise, we could make set_page_dirty_lock use a weird non-unplugging variant (although maybe that will upset Ken), but I'd rather look at simplification first ;)
> >>Yes. So set_page_dirty_lock is broken, right? >> > >yup. > > >>And the wait_on_page_stuff needs an inode ref. >>Also splice seems to have broken sync_page. >> > >Please describe the splice() problem which you've observed. >
sync_page wants to get either the current mapping, or a NULL one. The sync_page methods must then be able to handle running into a NULL mapping.
With splice, the mapping can change, so you can have the wrong sync_page callback run against the page.
> >>Well yes, writing to a page would be the main reason to set it dirty. >>Is splice broken as well? I'm not sure that it always has a ref on the >>inode when stealing a page. >> > >Whereabouts? >
The ->pin() calls in pipe_to_file and pipe_to_sendpage?
> >>It sounds like you think fixing the set_page_dirty_lock callers wouldn't >>be too difficult? I wouldn't know (although the ptrace one should be >>able to be turned into a set_page_dirty, because we're holding mmap_sem). >> > >No, I think it's damn impossible ;) > >get_user_pages() has gotten us a random pagecache page. How do we >non-racily get at the address_space prior to locking that page? > >I don't think we can. >
But the vma isn't going to disappear because mmap_sem is held; and the vma should hold a ref on the inode I think?
> >>You're sure about all other lock_page()rs? I'm not, given that >>set_page_dirty_lock got it so wrong. But you'd have a better idea than >>me. >> > >No, I'm not sure. > >However it is rare for the kernel to play with pagecache pages against >which the caller doesn't have an inode ref. Think: how did the caller look >up that page in the first place if not from the address_space in the first >place? > >- get_user_pages(): the current problem > >- page LRU: OK, uses trylock first. > >- pagetable walk?? >
Am I wrong about mmap_sem?
Anyway, it is possible that most of the problems could be solved by locking the page at the time of lookup, and unlocking it on completion/dirtying... it's just that that would be a bit of a task. Can we somehow add BUG_ONs to lock_page to ensure we've got an inode ref?
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