Re: [PATCH] AFS: Implement shared-writable mmap [try #2]

[Nick Piggin <nickpiggin@yahoo.com.au>]

David Howells wrote:
> Nick Piggin <nickpiggin@yahoo.com.au> wrote:
> 
> 
>>>I can't call invalidate_inode_pages() or similar because that might
>>>incorrectly kill one of B's writes (or someone else's writes); besides,
>>>the on-server file hasn't changed.
>>
>>Why would that kill anyone's writes?
> 
> 
> Because invalidate_inode_pages() forcibly removes the dirty flag from each page

It had better not. We use that sucker to nuke pagecache when we're trying to
reclaim inodes, for example...


>>>I can't as it can/would deadlock if called from prepare_write() in two
>>>different ways.
>>
>>Which ways? Are you talking about prepare_write being called from
>>page_mkwrite, or anywhere?
> 
> 
>  (1) prepare_write() is called with the target page locked and does not release
>      the lock.  The truncation routines lock the page prior to invalidating it.
>      Any truncation routine that skips locked pages is of no use.

You can drop the lock, do the invalidation, and return AOP_TRUNCATED_PAGE. The
new aops patches will provide a better solution, but that will work today.


>  (2) Consider a run of pages that make up a single write by one user.  Two
>      other writes from other users may be attempting to overwrite that run at
>      the same time.  Each of them would need to invalidate the other's locked
>      page(s).

See #1.


> Furthermore, the caller of prepare_write() probably won't take kindly to the
> page it's dealing with being evicted from the pagecache.

It's fine if you return AOP_TRUNCATED_PAGE.


>>More generally it sounds like a nasty thing to have a writeback cache if it
>>can become incoherent (due to dirty pages that subsequently cannot be
>>written back) without notification. Have you tried doing a write-through
>>one?
> 
> 
> How do you do a write-through cache for shared-writable mmap?

I just mean more generally. simple write(2) writes, for starters.

For shared writable mmap? I don't know... does POSIX require mmap data
to be coherent with read(2)/write(2)? ;)


>>You may be clearing PG_uptodate, but isn't there still an underlying problem
>>that you can have mappings to the page at that point? If that isn't a problem
>>for you, then I don't know why you would have to clear PG_uptodate at all.
> 
> 
> There might be, yes.  I guess I should ask the VM to nuke all PTEs to each of
> these pages too.

That's what the invalidate / truncate routines do.

-- 
SUSE Labs, Novell Inc.