Re: [PATCH] filemap: optimize order0 folio in filemap_map_pages

[Matthew Wilcox <willy@infradead.org>]

On Wed, Aug 20, 2025 at 09:10:56AM +0800, Jinjiang Tu wrote:
> 
> 在 2025/8/19 23:52, Matthew Wilcox 写道:
> > On Tue, Aug 19, 2025 at 10:06:53PM +0800, Jinjiang Tu wrote:
> > > There are two meaningless folio refcount update for order0 folio in
> > > filemap_map_pages(). First, filemap_map_order0_folio() adds folio refcount
> > > after the folio is mapped to pte. And then, filemap_map_pages() drops a
> > > refcount grabbed by next_uptodate_folio(). We could remain the refcount
> > > unchanged in this case.
> > > 
> > > With this patch, we can get 8% performance gain for lmbench testcase
> > > 'lat_pagefault -P 1 file', the size of file is 512M.
> > You don't explain why you move the folio_unlock() call
> 
> We should call folio_unlock() before folio_put(). In filemap_map_order0_folio(),
> if we doesn't set folio into pte, we should unlock and put folio.

I agree that folio_unlock() needs to be called before folio_put().
What I don't understand is why we need to delay folio_unlock() until
right before folio_put().  Can't we leave folio_unlock() where it
currently is and only move the folio_put()?