On 23 Apr 1999, Eric W. Biederman wrote:
> 
> LT> In fact, it already does for NFS, which uses PageLocked for exactly that
> LT> reason (to serialize read and write requests to the same page).
> 
> As I understand the NFS code it also tries using PageLocked to
> synchronize writes _to_ a page from user space.

Yes. It also uses it to synchronize various other write events, like the
per-page write queue etc.

Once that synchronization has been done, the thing is released: even
though there is a pending write. It's only a "data coherency lock", it's
not necessarily a "IO is pending on this" (you could use it for IO pending
too, but that would serialize everything: releasing the lock means that
you can gather up multiple write requests up until the time you decide to
actually physically push it out the door). 

> LT> For "valid to read", you only check Uptodate.  For any actual IO
> LT> (whether it's because you're going to actually _do_ the read or set up
> LT> some write), PageLocked should be used to synchronize.  NFS already does
> LT> this, and generic_file_read() already understands about it. 
> 
> generic_file_read is quite close but it always does wait_on_page.
> Not wrong, but not necessary if the page is Uptodate.

I thought I fixed that once already. Oh, well. Minor annoyance and
potentially a small performance hit (but it shouldn't really be a big
deal). 

> P.S. I'm looking at 2.2.5 so I may be a hair behind.

No, you're right. I know I looked at fixing it, but I obviously never
actually did. This was when I wanted to get 2.2 out.

		Linus


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