Re: [PATCH 01/13] mm: Update ptep_get_lockless()s comment

[Nadav Amit]

On Oct 30, 2022, at 10:00 PM, Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:

> So the current ordering rules are basically that we need to do
> set_page_dirty() *and* we need to flush the TLB's before dropping the
> page table lock. That's what gets us serialized with "mkclean”.

I understand. I am still not sure whether ordering the set_page_dirty() and
dropping the mapcount reference cannot suffice for the reclaim logic not to
free the buffers if the page is dirtied.

According to the code, shrink_page_list() first checks for folio_mapped()
and then for folio_test_dirty() to check whether pageout() is necessary.
IIUC, the buffers are not dropped up to this point and set_page_dirty()
would always set the page-struct dirty bit.

IOW: In shrink_page_list(), when we decide on whether to pageout(), we
should see whether the page is dirty (give or take smp_rmb()).

But this is an optimization and I do not know all the cases in which buffers
might be dropped. My intuition says that they cannot be dropped while
mapcount != 0, but I need to further explore it.