Re: [V9fs-developer] [PATCH 02/13] 9p: Tell the VFS that readpage was synchronous

Dominique Martinet
2020-09-18T12:31:53Z

Matthew Wilcox (Oracle) wrote on Thu, Sep 17, 2020:
> The 9p readpage implementation was already synchronous, so use
> AOP_UPDATED_PAGE to avoid cycling the page lock.
>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>

Acked-by: Dominique Martinet <asmadeus@codewreck.org>

(I assume it'll be merged together with the rest)

> > What I'm curious about is the page used to be both unlocked and put, but
> > now isn't either and the return value hasn't changed for the caller to
> > make a difference on write_begin / I don't see any code change in the
> > vfs to handle that.
> > What did I miss?
>
> The page cache is kind of subtle. The grab_cache_page_write_begin()
> will return a Locked page with an increased refcount. If it's Uptodate,
> that's exactly what we want, and we return it. If we have to read the
> page, readpage used to unlock the page before returning, and rather than
> re-lock it, we would drop the reference to the page and look it up again.
> It's possible that after dropping the lock on that page that the page
> was replaced in the page cache and so we'd get a different page.

Thanks for the explanation, I didn't realize the page already is
gotten/locked at the PageUptodate goto out.

> Anyway, now (unless fscache is involved), v9fs_fid_readpage will return
> the page without unlocking it. So we don't need to do the dance of
> dropping the lock, putting the refcount and looking the page back up
> again. We can just return the page. The VFS doesn't need a special
> return code because nothing has changed from the VFS's point of view --
> it asked you to get a page and you got the page.

Yes, looks good to me.

Cheers,
--
Dominique