Messages in this thread | | | Date | Sat, 16 Dec 2000 19:08:02 -0600 | From | Russell Cattelan <> | Subject | Re: Test12 ll_rw_block error. |
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"Stephen C. Tweedie" wrote:
> Hi, > > On Fri, Dec 15, 2000 at 02:00:19AM -0500, Alexander Viro wrote: > > On Thu, 14 Dec 2000, Linus Torvalds wrote: > > > > Just one: any fs that really cares about completion callback is very likely > > to be picky about the requests ordering. So sync_buffers() is very unlikely > > to be useful anyway. > > > > In that sense we really don't have anonymous buffers here. I seriously > > suspect that "unrealistic" assumption is not unrealistic at all. I'm > > not sufficiently familiar with XFS code to say for sure, but... > > Right. ext3 and reiserfs just want to submit their own IOs when it > comes to the journal. (At least in ext3, already-journaled buffers > can be written back by the VM freely.) It's a matter of telling the > fs when that should start. > > > What we really need is a way for VFS/VM to pass the pressure on filesystem. > > That's it. If fs wants unusual completions for requests - let it have its > > own queueing mechanism and submit these requests when it finds that convenient. > > There is a very clean way of doing this with address spaces. It's > something I would like to see done properly for 2.5: eliminate all > knowledge of buffer_heads from the VM layer. It would be pretty > simple to remove page->buffers completely and replace it with a > page->private pointer, owned by whatever address_space controlled the > page. Instead of trying to unmap and flush buffers on the page > directly, these operations would become address_space operations.
Yes this is a lot of what page buf would like to do eventually. Have the VM system pressure page_buf for pages which would then be able to intelligently call the file system to free up cached pages. A big part of getting Delay Alloc to not completely consume all the system pages, is being told when it's time to start really allocating disk space and push pages out.
> > > We could still provide the standard try_to_free_buffers() and > unmap_underlying_metadata() functions to operate on the per-page > buffer_head lists, and existing filesystems would only have to point > their address_space "private metadata" operations at the generic > functions. However, a filesystem which had additional ordering > constraints could then intercept the flush or writeback calls from the > VM and decide on its own how best to honour the VM pressure. > > This even works both for hashed and unhashed anonymous buffers, *if* > you allow the filesystem to attach all of its hashed buffers buffers > to an address_space of its own rather than having the buffer cache > pool all such buffers together. > > --Stephen
-- Russell Cattelan cattelan@thebarn.com
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