On Thu, Sep 17, 2020 at 11:23 AM Matthew Wilcox <willy@xxxxxxxxxxxxx> wrote:
>
> Something like taking
> the i_mmap_lock_read(file->f_mapping) in filemap_fault, then adding a
> new VM_FAULT_I_MMAP_LOCKED bit so that do_read_fault() and friends add:
>
> if (ret & VM_FAULT_I_MMAP_LOCKED)
> i_mmap_unlock_read(vmf->vma->vm_file->f_mapping);
> else
> unlock_page(page);
>
> ... want me to turn that into a real patch?
I can't guarantee it's the right model - it does worry me how many
places we might get that i_mmap_rwlock, and how long we migth hold it
for writing, and what deadlocks it might cause when we take it for
reading in the page fault path.
But I think it might be very interesting as a benchmark patch and a
trial balloon. Maybe it "just works".
I would _love_ for the page lock itself to be only (or at least
_mainly_) about the actual IO synchronization on the page.
That was the origin of it, the whole "protect all the complex state of
a page" behavior kind of grew over time, since it was the only
per-page lock we had.
Linus
