On Wed, Aug 14, 2024 at 04:44:24PM +0100, Matthew Wilcox (Oracle) wrote:
Hi Matthew,
> I believe the test for PageDirty() is no longer needed. The
> commit adding it was abf09bed3cce with the rationale that this
> avoided faults for tmpfs and shmem pages. shmem does not mark
> newly allocated folios as dirty since 2016 (commit 75edd345e8ed)
> so this test has been ineffective since then.
The PageDirty() test you suggest to remove is still entered.
I initially thought that test could also be useful for other
architectures as an optimization, but at least one path we
take for shmem mapping is raising eyebrow, because it is a
read accesss:
handle_pte_fault() -> do_pte_missing() -> do_fault() ->
do_read_fault() -> finish_fault() -> set_pte_range() -> mk_pte()
A read fault causing the PTE dirtifying is something strange
and your patch alone could be a nice cleanup.
As other architectures do not do such a trick suggests that
mk_pte() + pte_mkdirty() is called from the same handler
or pte_mkdirty() is expected to be called from a follow-up
write handler.
I could not identify locations where that would not be the case,
but may be you know?
...
> -static inline pte_t mk_pte(struct page *page, pgprot_t pgprot)
> -{
> - unsigned long physpage = page_to_phys(page);
> - pte_t __pte = mk_pte_phys(physpage, pgprot);
> -
> - if (pte_write(__pte) && PageDirty(page))
> - __pte = pte_mkdirty(__pte);
> - return __pte;
> -}
> -#define mk_pte mk_pte
Thanks!
