Messages in this thread | | | Date | Mon, 2 Jun 2008 09:43:05 +0100 (BST) | From | Hugh Dickins <> | Subject | Re: sync_file_range(SYNC_FILE_RANGE_WRITE) blocks? |
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On Sun, 1 Jun 2008, Andrew Morton wrote: > On Mon, 2 Jun 2008 01:00:40 +0200 Pavel Machek <pavel@suse.cz> wrote: > > > How about this: > > > > > > - Add a new SYNC_FILE_RANGE_NON_BLOCKING > > > > > > - If userspace set that flag, turn on writeback_control.nonblocking > > > in __filemap_fdatawrite_range(). > > > > > > - test it a lot. > > > > Works for me. Is the expectation that I code this? I can certainly > > provide testing ;-). > > Something like this:
Though this fits very easily into the current kernel implementation, I don't think it's the right interface for userspace.
If we do go this kind of a way, then I'd say SYNC_FILE_RANGE_NON_BLOCKING needs to tell the caller how far it got before giving up, rather than just success or failure. Why? um, um, because it feels right; and would help the caller help the kernel by not overloading it with needlessly repeated loop ranges - any stronger reasons? But sync_file_range() was defined to return int rather than ssize_t, so that becomes awkward.
Never mind, I don't think it is the right way anyway. We don't need additions to the existing sync_file_range() interface, we just need it to behave as naive people like Pavel and I expected it to behave in the first place: SYNC_FILE_RANGE_WRITE should be nonblocking (with respect to queue congestion, and maybe page locking also).
I was imagining that where the existing nonblocking code just gives up, the SYNC_FILE_RANGE_WRITE case should schedule the remaining work to be done a little later: possibly by poking and/or leaving info for pdflush.
I guess there may be some resource fairness issues, with either approach: it ought not to be unreasonable for a process to proceed by writing a page then SYNC_FILE_RANGE_WRITing that page, page by page, should it?
But once we claim nonblocking at the user interface, I expect we'll come up against the raciness in the current nonblocking treatment: just because bdi is not congested when it's tested doesn't mean we won't block when the write is submitted. Perhaps a BIO_RW_NONBLOCK could fix that up?
Hugh
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