On Tue, Jun 27, 2023 at 3:41 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
On 27/06/2023 09:29, Yu Zhao wrote:
On Tue, Jun 27, 2023 at 1:21 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
On 27/06/2023 02:55, Yu Zhao wrote:
On Mon, Jun 26, 2023 at 11:14 AM Ryan Roberts <ryan.roberts@xxxxxxx> wrote:
In preparation for extending vma_alloc_zeroed_movable_folio() to
allocate a arbitrary order folio, expose clear_huge_page()
unconditionally, so that it can be used to zero the allocated folio in
the generic implementation of vma_alloc_zeroed_movable_folio().
Signed-off-by: Ryan Roberts <ryan.roberts@xxxxxxx>
---
include/linux/mm.h | 3 ++-
mm/memory.c | 2 +-
2 files changed, 3 insertions(+), 2 deletions(-)
diff --git a/include/linux/mm.h b/include/linux/mm.h
index 7f1741bd870a..7e3bf45e6491 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3684,10 +3684,11 @@ enum mf_action_page_type {
*/
extern const struct attribute_group memory_failure_attr_group;
-#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
extern void clear_huge_page(struct page *page,
unsigned long addr_hint,
unsigned int pages_per_huge_page);
+
+#if defined(CONFIG_TRANSPARENT_HUGEPAGE) || defined(CONFIG_HUGETLBFS)
We might not want to depend on THP eventually. Right now, we still
have to, unless splitting is optional, which seems to contradict
06/10. (deferred_split_folio() is a nop without THP.)
Yes, I agree - for large anon folios to work, we depend on THP. But I don't
think that helps us here.
In the next patch, I give vma_alloc_zeroed_movable_folio() an extra `order`
parameter. So the generic/default version of the function now needs a way to
clear a compound page.
I guess I could do something like:
static inline
struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
unsigned long vaddr, gfp_t gfp, int order)
{
struct folio *folio;
folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE | gfp,
order, vma, vaddr, false);
if (folio) {
#ifdef CONFIG_LARGE_FOLIO
clear_huge_page(&folio->page, vaddr, 1U << order);
#else
BUG_ON(order != 0);
clear_user_highpage(&folio->page, vaddr);
#endif
}
return folio;
}
But that's pretty messy and there's no reason why other users might come along
that pass order != 0 and will be surprised by the BUG_ON.
#ifdef CONFIG_LARGE_ANON_FOLIO // depends on CONFIG_TRANSPARENT_HUGE_PAGE
struct folio *alloc_anon_folio(struct vm_area_struct *vma, unsigned
long vaddr, int order)
{
// how do_huge_pmd_anonymous_page() allocs and clears
vma_alloc_folio(..., *true*);
This controls the mem allocation policy (see mempolicy.c::vma_alloc_folio()) not
clearing. Clearing is done in __do_huge_pmd_anonymous_page():
clear_huge_page(page, vmf->address, HPAGE_PMD_NR);
Sorry for rushing this previously. This is what I meant. The #ifdef
makes it safe to use clear_huge_page() without 01/10. I highlighted
the last parameter to vma_alloc_folio() only because it's different
from what you chose (not implying it clears the folio).
}
#else
#define alloc_anon_folio(vma, addr, order)
vma_alloc_zeroed_movable_folio(vma, addr)
#endif
Sorry I don't get this at all... If you are suggesting to bypass
vma_alloc_zeroed_movable_folio() entirely for the LARGE_ANON_FOLIO case
Correct.
I don't
think that works because the arch code adds its own gfp flags there. For
example, arm64 adds __GFP_ZEROTAGS for VM_MTE VMAs.
I think it's the opposite: it should be safer to reuse the THP code because
1. It's an existing case that has been working for PMD_ORDER folios
mapped by PTEs, and it's an arch-independent API which would be easier
to review.
2. Use vma_alloc_zeroed_movable_folio() for large folios is a *new*
case. It's an arch-*dependent* API which I have no idea what VM_MTE
does (should do) to large folios and don't plan to answer that for
now.
Perhaps we can do away with an arch-owned vma_alloc_zeroed_movable_folio() and
replace it with a new arch_get_zeroed_movable_gfp_flags() then
alloc_anon_folio() add in those flags?
But I still think the cleanest, simplest change is just to unconditionally
expose clear_huge_page() as I've done it.
The fundamental choice there as I see it is to whether the first step
of large anon folios should lean toward the THP code base or the base
page code base (I'm a big fan of the answer "Neither -- we should
create something entirely new instead"). My POV is that the THP code
base would allow us to move faster, since it's proven to work for a
very similar case (PMD_ORDER folios mapped by PTEs).
