Re: [PATCH v2] ext4: check folio uptodate state in ext4_page_mkwrite()

[Matthew Wilcox <willy@infradead.org>]

You snipped out all the context when adding me to the cc, and I'm on
holiday until after Plumbers, so I'm disinclined to go looking for
context.

On Wed, Dec 03, 2025 at 10:46:57AM -0500, Theodore Tso wrote:
> My main concern with your patch is folio_lock() is *incredibly*
> heavyweight and is going to be a real scalability concern if we need
> to take it every single time we need to make a page writeable.
> 
> So could we perhaps do something like this?  So the first question is
> do we need to take the lock at all?  I'm not sure we need to worry
> about the case where the page is not uptodate because we're racing
> with the page being brought into memory; if we that could happen under
> normal circumstances we would be triggering the warning even without
> these situations such as a delayed allocaiton write failing due to a
> corrupted file system image.   So can we just do this?
> 
> 	if (!folio_test_uptodate(folio)) {
> 		ret = VM_FAULT_SIGBUS;
> 		goto out;
> 	}
> 
> If it is legitmate that ext4_page_mkwrite() could be called while the
> page is still being read in (and again, I don't think it is), then we
> could do something like this:
> 
> 	if (!folio_test_uptodate(folio)) {
> 		folio_lock(folio);
> 		if (!folio_test_uptodate(folio)) {
> 			folio_unlock(folio);
> 			ret = VM_FAULT_SIGBUS;
> 			goto out;
> 		}
> 		folio_unlock(folio);
> 	}
> 
> Matthew, as the page cache maintainer, do we actually need this extra
> rigamarole.  Or can we just skip taking the lock before checking to
> see if the folio is uptodate in ext4_page_mkwrite()?
> 
> 							- Ted