On 14/07/2023 18:17, Yu Zhao wrote:

On Fri, Jul 14, 2023 at 10:17 AM Ryan Roberts [off-list ref] wrote:

quoted

Introduce FLEXIBLE_THP feature, which allows anonymous memory to be
allocated in large folios of a determined order. All pages of the large
folio are pte-mapped during the same page fault, significantly reducing
the number of page faults. The number of per-page operations (e.g. ref
counting, rmap management lru list management) are also significantly
reduced since those ops now become per-folio.

The new behaviour is hidden behind the new FLEXIBLE_THP Kconfig, which
defaults to disabled for now; The long term aim is for this to defaut to
enabled, but there are some risks around internal fragmentation that
need to be better understood first.

When enabled, the folio order is determined as such: For a vma, process
or system that has explicitly disabled THP, we continue to allocate
order-0. THP is most likely disabled to avoid any possible internal
fragmentation so we honour that request.

Otherwise, the return value of arch_wants_pte_order() is used. For vmas
that have not explicitly opted-in to use transparent hugepages (e.g.
where thp=madvise and the vma does not have MADV_HUGEPAGE), then
arch_wants_pte_order() is limited by the new cmdline parameter,
`flexthp_unhinted_max`. This allows for a performance boost without
requiring any explicit opt-in from the workload while allowing the
sysadmin to tune between performance and internal fragmentation.

arch_wants_pte_order() can be overridden by the architecture if desired.
Some architectures (e.g. arm64) can coalsece TLB entries if a contiguous
set of ptes map physically contigious, naturally aligned memory, so this
mechanism allows the architecture to optimize as required.

If the preferred order can't be used (e.g. because the folio would
breach the bounds of the vma, or because ptes in the region are already
mapped) then we fall back to a suitable lower order; first
PAGE_ALLOC_COSTLY_ORDER, then order-0.

Signed-off-by: Ryan Roberts <ryan.roberts@arm.com>
---
 .../admin-guide/kernel-parameters.txt         |  10 +
 mm/Kconfig                                    |  10 +
 mm/memory.c                                   | 187 ++++++++++++++++--
 3 files changed, 190 insertions(+), 17 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index a1457995fd41..405d624e2191 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt

@@ -1497,6 +1497,16 @@
                        See Documentation/admin-guide/sysctl/net.rst for
                        fb_tunnels_only_for_init_ns

+       flexthp_unhinted_max=
+                       [KNL] Requires CONFIG_FLEXIBLE_THP enabled. The maximum
+                       folio size that will be allocated for an anonymous vma
+                       that has neither explicitly opted in nor out of using
+                       transparent hugepages. The size must be a power-of-2 in
+                       the range [PAGE_SIZE, PMD_SIZE). A larger size improves
+                       performance by reducing page faults, while a smaller
+                       size reduces internal fragmentation. Default: max(64K,
+                       PAGE_SIZE). Format: size[KMG].
+

Let's split this parameter into a separate patch.

Ha - I had it as a separate patch originally, but thought you'd ask for it to be
a single patch so squashed it ;-). I can separate it again, no problem.

And I'm going to ask many questions about it (I can live with a sysctl
parameter but this boot parameter is unacceptable to me).

Please do. Hopefully the thread with DavidH against v2 gives the rationale. Care
to elaborate on why its unacceptable?

quoted

diff --git a/mm/memory.c b/mm/memory.c
index 01f39e8144ef..e8bc729efb9d 100644
--- a/mm/memory.c
+++ b/mm/memory.c

@@ -4050,6 +4050,148 @@ vm_fault_t do_swap_page(struct vm_fault *vmf)
        return ret;
 }

+static bool vmf_pte_range_changed(struct vm_fault *vmf, int nr_pages)
+{
+       int i;
+
+       if (nr_pages == 1)
+               return vmf_pte_changed(vmf);
+
+       for (i = 0; i < nr_pages; i++) {
+               if (!pte_none(ptep_get_lockless(vmf->pte + i)))
+                       return true;
+       }
+
+       return false;
+}
+
+#ifdef CONFIG_FLEXIBLE_THP
+static int flexthp_unhinted_max_order =
+               ilog2(SZ_64K > PAGE_SIZE ? SZ_64K : PAGE_SIZE) - PAGE_SHIFT;
+
+static int __init parse_flexthp_unhinted_max(char *s)
+{
+       unsigned long long size = memparse(s, NULL);
+
+       if (!is_power_of_2(size) || size < PAGE_SIZE || size > PMD_SIZE) {
+               pr_warn("flexthp: flexthp_unhinted_max=%s must be power-of-2 between PAGE_SIZE (%lu) and PMD_SIZE (%lu), ignoring\n",
+                       s, PAGE_SIZE, PMD_SIZE);
+               return 1;
+       }
+
+       flexthp_unhinted_max_order = ilog2(size) - PAGE_SHIFT;
+
+       /* THP machinery requires at least 3 struct pages for meta data. */
+       if (flexthp_unhinted_max_order == 1)
+               flexthp_unhinted_max_order--;
+
+       return 1;
+}
+
+__setup("flexthp_unhinted_max=", parse_flexthp_unhinted_max);
+
+static int anon_folio_order(struct vm_area_struct *vma)
+{
+       int order;
+
+       /*
+        * If THP is explicitly disabled for either the vma, the process or the
+        * system, then this is very likely intended to limit internal
+        * fragmentation; in this case, don't attempt to allocate a large
+        * anonymous folio.
+        *
+        * Else, if the vma is eligible for thp, allocate a large folio of the
+        * size preferred by the arch. Or if the arch requested a very small
+        * size or didn't request a size, then use PAGE_ALLOC_COSTLY_ORDER,
+        * which still meets the arch's requirements but means we still take
+        * advantage of SW optimizations (e.g. fewer page faults).
+        *
+        * Finally if thp is enabled but the vma isn't eligible, take the
+        * arch-preferred size and limit it to the flexthp_unhinted_max cmdline
+        * parameter. This allows a sysadmin to tune performance vs internal
+        * fragmentation.
+        */
+
+       if ((vma->vm_flags & VM_NOHUGEPAGE) ||
+           test_bit(MMF_DISABLE_THP, &vma->vm_mm->flags) ||
+           !hugepage_flags_enabled())
+               order = 0;
+       else {
+               order = max(arch_wants_pte_order(), PAGE_ALLOC_COSTLY_ORDER);
+
+               if (!hugepage_vma_check(vma, vma->vm_flags, false, true, true))
+                       order = min(order, flexthp_unhinted_max_order);
+       }
+
+       return order;
+}
+
+static int alloc_anon_folio(struct vm_fault *vmf, struct folio **folio)
+{
+       int i;
+       gfp_t gfp;
+       pte_t *pte;
+       unsigned long addr;
+       struct vm_area_struct *vma = vmf->vma;
+       int prefer = anon_folio_order(vma);
+       int orders[] = {
+               prefer,
+               prefer > PAGE_ALLOC_COSTLY_ORDER ? PAGE_ALLOC_COSTLY_ORDER : 0,
+               0,
+       };
+
+       *folio = NULL;
+
+       if (vmf_orig_pte_uffd_wp(vmf))
+               goto fallback;
+
+       for (i = 0; orders[i]; i++) {
+               addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << orders[i]);
+               if (addr >= vma->vm_start &&
+                   addr + (PAGE_SIZE << orders[i]) <= vma->vm_end)
+                       break;
+       }
+
+       if (!orders[i])
+               goto fallback;
+
+       pte = pte_offset_map(vmf->pmd, vmf->address & PMD_MASK);
+       if (!pte)
+               return -EAGAIN;

It would be a bug if this happens. So probably -EINVAL?

Not sure what you mean? Hugh Dickins' series that went into v6.5-rc1 makes it
possible for pte_offset_map() to fail (if I understood correctly) and we have to
handle this. The intent is that we will return from the fault without making any
change, then we will refault and try again.

quoted

+
+       for (; orders[i]; i++) {
+               addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << orders[i]);
+               vmf->pte = pte + pte_index(addr);
+               if (!vmf_pte_range_changed(vmf, 1 << orders[i]))
+                       break;
+       }
+
+       vmf->pte = NULL;
+       pte_unmap(pte);
+
+       gfp = vma_thp_gfp_mask(vma);
+
+       for (; orders[i]; i++) {
+               addr = ALIGN_DOWN(vmf->address, PAGE_SIZE << orders[i]);
+               *folio = vma_alloc_folio(gfp, orders[i], vma, addr, true);
+               if (*folio) {
+                       clear_huge_page(&(*folio)->page, addr, 1 << orders[i]);
+                       return 0;
+               }
+       }
+
+fallback:
+       *folio = vma_alloc_zeroed_movable_folio(vma, vmf->address);
+       return *folio ? 0 : -ENOMEM;
+}
+#else
+static inline int alloc_anon_folio(struct vm_fault *vmf, struct folio **folio)

Drop "inline" (it doesn't do anything in .c).

There are 38 instances of inline in memory.c alone, so looks like a well used
convention, even if the compiler may choose to ignore. Perhaps you can educate
me; what's the benefit of dropping it?

The rest looks good to me.

Great - just incase it wasn't obvious, I decided not to overwrite vmf->address
with the aligned version, as you suggested, for 2 reasons; 1) address is const
in the struct, so would have had to change that. 2) there is a uffd path that
can be taken after the vmf->address fixup would have occured and the path
consumes that member, so it would have had to be un-fixed-up making it more
messy than the way I opted for.

Thanks for the quick review as always!


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