[RFC PATCH 14/26] hugetlb: add HGM support for hugetlb_fault and hugetlb_no_page

From: James Houghton
Date: Fri Jun 24 2022 - 13:38:38 EST

This CL is the first main functional HugeTLB change. Together, these
changes allow the HugeTLB fault path to handle faults on HGM-enabled
VMAs. The two main behaviors that can be done now:
1. Faults can be passed to handle_userfault. (Userspace will want to
use UFFD_FEATURE_REAL_ADDRESS to get the real address to know which
region they should be call UFFDIO_CONTINUE on later.)
2. Faults on pages that have been partially mapped (and userfaultfd is
not being used) will get mapped at the largest possible size.
For example, if a 1G page has been partially mapped at 2M, and we
fault on an unmapped 2M section, hugetlb_no_page will create a 2M
PMD to map the faulting address.

This commit does not handle hugetlb_wp right now, and it doesn't handle
HugeTLB page migration and swap entries.

Signed-off-by: James Houghton <jthoughton@xxxxxxxxxx>
---
include/linux/hugetlb.h | 12 ++++
mm/hugetlb.c | 121 +++++++++++++++++++++++++++++++---------
2 files changed, 106 insertions(+), 27 deletions(-)

diff --git a/include/linux/hugetlb.h b/include/linux/hugetlb.h
index 321f5745d87f..ac4ac8fbd901 100644
--- a/include/linux/hugetlb.h
+++ b/include/linux/hugetlb.h
@@ -1185,6 +1185,9 @@ enum split_mode {
#ifdef CONFIG_HUGETLB_HIGH_GRANULARITY_MAPPING
/* If HugeTLB high-granularity mappings are enabled for this VMA. */
bool hugetlb_hgm_enabled(struct vm_area_struct *vma);
+int hugetlb_alloc_largest_pte(struct hugetlb_pte *hpte, struct mm_struct *mm,
+ struct vm_area_struct *vma, unsigned long start,
+ unsigned long end);
int huge_pte_alloc_high_granularity(struct hugetlb_pte *hpte,
struct mm_struct *mm,
struct vm_area_struct *vma,
@@ -1197,6 +1200,15 @@ static inline bool hugetlb_hgm_enabled(struct vm_area_struct *vma)
{
return false;
}
+
+static inline
+int hugetlb_alloc_largest_pte(struct hugetlb_pte *hpte, struct mm_struct *mm,
+ struct vm_area_struct *vma, unsigned long start,
+ unsigned long end)
+{
+ BUG();
+}
+
static inline int huge_pte_alloc_high_granularity(struct hugetlb_pte *hpte,
struct mm_struct *mm,
struct vm_area_struct *vma,
diff --git a/mm/hugetlb.c b/mm/hugetlb.c
index 6e0c5fbfe32c..da30621656b8 100644
--- a/mm/hugetlb.c
+++ b/mm/hugetlb.c
@@ -5605,18 +5605,24 @@ static inline vm_fault_t hugetlb_handle_userfault(struct vm_area_struct *vma,
static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
struct vm_area_struct *vma,
struct address_space *mapping, pgoff_t idx,
- unsigned long address, pte_t *ptep,
+ unsigned long address, struct hugetlb_pte *hpte,
pte_t old_pte, unsigned int flags)
{
struct hstate *h = hstate_vma(vma);
vm_fault_t ret = VM_FAULT_SIGBUS;
int anon_rmap = 0;
unsigned long size;
- struct page *page;
+ struct page *page, *subpage;
pte_t new_pte;
spinlock_t *ptl;
unsigned long haddr = address & huge_page_mask(h);
+ unsigned long haddr_hgm = address & hugetlb_pte_mask(hpte);
bool new_page, new_pagecache_page = false;
+ /*
+ * This page is getting mapped for the first time, in which case we
+ * want to increment its mapcount.
+ */
+ bool new_mapping = hpte->shift == huge_page_shift(h);

/*
* Currently, we are forced to kill the process in the event the
@@ -5665,9 +5671,9 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
* here. Before returning error, get ptl and make
* sure there really is no pte entry.
*/
- ptl = huge_pte_lock(h, mm, ptep);
+ ptl = hugetlb_pte_lock(mm, hpte);
ret = 0;
- if (huge_pte_none(huge_ptep_get(ptep)))
+ if (hugetlb_pte_none(hpte))
ret = vmf_error(PTR_ERR(page));
spin_unlock(ptl);
goto out;
@@ -5731,18 +5737,25 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
vma_end_reservation(h, vma, haddr);
}

- ptl = huge_pte_lock(h, mm, ptep);
+ ptl = hugetlb_pte_lock(mm, hpte);
ret = 0;
/* If pte changed from under us, retry */
- if (!pte_same(huge_ptep_get(ptep), old_pte))
+ if (!pte_same(hugetlb_ptep_get(hpte), old_pte))
goto backout;

- if (anon_rmap) {
- ClearHPageRestoreReserve(page);
- hugepage_add_new_anon_rmap(page, vma, haddr);
- } else
- page_dup_file_rmap(page, true);
- new_pte = make_huge_pte(vma, page, ((vma->vm_flags & VM_WRITE)
+ if (new_mapping) {
+ /* Only increment this page's mapcount if we are mapping it
+ * for the first time.
+ */
+ if (anon_rmap) {
+ ClearHPageRestoreReserve(page);
+ hugepage_add_new_anon_rmap(page, vma, haddr);
+ } else
+ page_dup_file_rmap(page, true);
+ }
+
+ subpage = hugetlb_find_subpage(h, page, haddr_hgm);
+ new_pte = make_huge_pte(vma, subpage, ((vma->vm_flags & VM_WRITE)
&& (vma->vm_flags & VM_SHARED)));
/*
* If this pte was previously wr-protected, keep it wr-protected even
@@ -5750,12 +5763,13 @@ static vm_fault_t hugetlb_no_page(struct mm_struct *mm,
*/
if (unlikely(pte_marker_uffd_wp(old_pte)))
new_pte = huge_pte_wrprotect(huge_pte_mkuffd_wp(new_pte));
- set_huge_pte_at(mm, haddr, ptep, new_pte);
+ set_huge_pte_at(mm, haddr_hgm, hpte->ptep, new_pte);

- hugetlb_count_add(pages_per_huge_page(h), mm);
+ hugetlb_count_add(hugetlb_pte_size(hpte) / PAGE_SIZE, mm);
if ((flags & FAULT_FLAG_WRITE) && !(vma->vm_flags & VM_SHARED)) {
+ BUG_ON(hugetlb_pte_size(hpte) != huge_page_size(h));
/* Optimization, do the COW without a second fault */
- ret = hugetlb_wp(mm, vma, address, ptep, flags, page, ptl);
+ ret = hugetlb_wp(mm, vma, address, hpte->ptep, flags, page, ptl);
}

spin_unlock(ptl);
@@ -5816,11 +5830,15 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
u32 hash;
pgoff_t idx;
struct page *page = NULL;
+ struct page *subpage = NULL;
struct page *pagecache_page = NULL;
struct hstate *h = hstate_vma(vma);
struct address_space *mapping;
int need_wait_lock = 0;
unsigned long haddr = address & huge_page_mask(h);
+ unsigned long haddr_hgm;
+ bool hgm_enabled = hugetlb_hgm_enabled(vma);
+ struct hugetlb_pte hpte;

ptep = huge_pte_offset(mm, haddr, huge_page_size(h));
if (ptep) {
@@ -5866,11 +5884,22 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
hash = hugetlb_fault_mutex_hash(mapping, idx);
mutex_lock(&hugetlb_fault_mutex_table[hash]);

- entry = huge_ptep_get(ptep);
+ hugetlb_pte_populate(&hpte, ptep, huge_page_shift(h));
+
+ if (hgm_enabled) {
+ ret = hugetlb_walk_to(mm, &hpte, address,
+ PAGE_SIZE, /*stop_at_none=*/true);
+ if (ret) {
+ ret = vmf_error(ret);
+ goto out_mutex;
+ }
+ }
+
+ entry = hugetlb_ptep_get(&hpte);
/* PTE markers should be handled the same way as none pte */
- if (huge_pte_none_mostly(entry)) {
- ret = hugetlb_no_page(mm, vma, mapping, idx, address, ptep,
- entry, flags);
+ if (hugetlb_pte_none_mostly(&hpte)) {
+ ret = hugetlb_no_page(mm, vma, mapping, idx, address, &hpte,
+ entry, flags);
goto out_mutex;
}

@@ -5908,14 +5937,17 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
vma, haddr);
}

- ptl = huge_pte_lock(h, mm, ptep);
+ ptl = hugetlb_pte_lock(mm, &hpte);

/* Check for a racing update before calling hugetlb_wp() */
- if (unlikely(!pte_same(entry, huge_ptep_get(ptep))))
+ if (unlikely(!pte_same(entry, hugetlb_ptep_get(&hpte))))
goto out_ptl;

+ /* haddr_hgm is the base address of the region that hpte maps. */
+ haddr_hgm = address & hugetlb_pte_mask(&hpte);
+
/* Handle userfault-wp first, before trying to lock more pages */
- if (userfaultfd_wp(vma) && huge_pte_uffd_wp(huge_ptep_get(ptep)) &&
+ if (userfaultfd_wp(vma) && huge_pte_uffd_wp(hugetlb_ptep_get(&hpte)) &&
(flags & FAULT_FLAG_WRITE) && !huge_pte_write(entry)) {
struct vm_fault vmf = {
.vma = vma,
@@ -5939,7 +5971,8 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
* pagecache_page, so here we need take the former one
* when page != pagecache_page or !pagecache_page.
*/
- page = pte_page(entry);
+ subpage = pte_page(entry);
+ page = compound_head(subpage);
if (page != pagecache_page)
if (!trylock_page(page)) {
need_wait_lock = 1;
@@ -5950,7 +5983,8 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,

if (flags & (FAULT_FLAG_WRITE|FAULT_FLAG_UNSHARE)) {
if (!huge_pte_write(entry)) {
- ret = hugetlb_wp(mm, vma, address, ptep, flags,
+ BUG_ON(hugetlb_pte_size(&hpte) != huge_page_size(h));
+ ret = hugetlb_wp(mm, vma, address, hpte.ptep, flags,
pagecache_page, ptl);
goto out_put_page;
} else if (likely(flags & FAULT_FLAG_WRITE)) {
@@ -5958,9 +5992,9 @@ vm_fault_t hugetlb_fault(struct mm_struct *mm, struct vm_area_struct *vma,
}
}
entry = pte_mkyoung(entry);
- if (huge_ptep_set_access_flags(vma, haddr, ptep, entry,
+ if (huge_ptep_set_access_flags(vma, haddr_hgm, hpte.ptep, entry,
flags & FAULT_FLAG_WRITE))
- update_mmu_cache(vma, haddr, ptep);
+ update_mmu_cache(vma, haddr_hgm, hpte.ptep);
out_put_page:
if (page != pagecache_page)
unlock_page(page);
@@ -6951,7 +6985,8 @@ pte_t *huge_pte_alloc(struct mm_struct *mm, struct vm_area_struct *vma,
pte = (pte_t *)pmd_alloc(mm, pud, addr);
}
}
- BUG_ON(pte && pte_present(*pte) && !pte_huge(*pte));
+ if (!hugetlb_hgm_enabled(vma))
+ BUG_ON(pte && pte_present(*pte) && !pte_huge(*pte));

return pte;
}
@@ -7057,6 +7092,38 @@ static unsigned int __shift_for_hstate(struct hstate *h)
(tmp_h) <= &hstates[hugetlb_max_hstate]; \
(tmp_h)++)

+/*
+ * Allocate a HugeTLB PTE that maps as much of [start, end) as possible with a
+ * single page table entry. The allocated HugeTLB PTE is returned in hpte.
+ */
+int hugetlb_alloc_largest_pte(struct hugetlb_pte *hpte, struct mm_struct *mm,
+ struct vm_area_struct *vma, unsigned long start,
+ unsigned long end)
+{
+ struct hstate *h = hstate_vma(vma), *tmp_h;
+ unsigned int shift;
+ int ret;
+
+ for_each_hgm_shift(h, tmp_h, shift) {
+ unsigned long sz = 1UL << shift;
+
+ if (!IS_ALIGNED(start, sz) || start + sz > end)
+ continue;
+ ret = huge_pte_alloc_high_granularity(hpte, mm, vma, start,
+ shift, HUGETLB_SPLIT_NONE,
+ /*write_locked=*/false);
+ if (ret)
+ return ret;
+
+ if (hpte->shift > shift)
+ return -EEXIST;
+
+ BUG_ON(hpte->shift != shift);
+ return 0;
+ }
+ return -EINVAL;
+}
+
/*
* Given a particular address, split the HugeTLB PTE that currently maps it
* so that, for the given address, the PTE that maps it is `desired_shift`.
--
2.37.0.rc0.161.g10f37bed90-goog