On Tue, Feb 9, 2021 at 12:21 PM Matthew Wilcox [off-list ref] wrote:

Yeah, I have trouble with the private2 vs fscache bit too.  I've been
trying to persuade David that he doesn't actually need an fscache
bit at all; he can just increment the page's refcount to prevent it
from being freed while he writes data to the cache.

Does the code not hold a refcount already?

Honestly, the fact that writeback doesn't take a refcount, and then
has magic "if writeback is set, don't free" code in other parts of the
VM layer has been a problem already, when the wakeup ended up
"leaking" from a previous page to a new allocation.

I very much hope the fscache bit does not make similar mistakes,
because the rest of the VM will _not_ have special "if fscache is set,
then we won't do X" the way we do for writeback.

So I think the fscache code needs to hold a refcount regardless, and
that the fscache bit is set the page has to have a reference.

So what are the current lifetime rules for the fscache bit?

             Linus