On 2022/7/26 21:09, Matthew Wilcox wrote:
On Tue, Jul 26, 2022 at 08:43:15PM +0800, Liu Zixian wrote:
Transparent hugepage of tmpfs is useful to improve TLB miss, but
it will be split during cow memory fault.
That's intentional. Possibly misguided, but there's a tradeoff to
be made between memory consumption and using large pages.
This will happen if we mprotect and rewrite code segment (which is
private file map) to hotpatch a running process.
We can avoid the splitting by adding a huge_fault function.
Signed-off-by: Liu Zixian <liuzixian4@xxxxxxxxxx>
---
mm/shmem.c | 46 ++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 46 insertions(+)
diff --git a/mm/shmem.c b/mm/shmem.c
index a6f565308..12b2b5140 100644
--- a/mm/shmem.c
+++ b/mm/shmem.c
@@ -2120,6 +2120,51 @@ static vm_fault_t shmem_fault(struct vm_fault *vmf)
return ret;
}
+static vm_fault_t shmem_huge_fault(struct vm_fault *vmf, enum page_entry_size pe_size)
+{
+ vm_fault_t ret = VM_FAULT_FALLBACK;
+ unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
+ struct page *old_page, *new_page;
+ int gfp_flags = GFP_HIGHUSER_MOVABLE | __GFP_COMP;
There are many vmf->vma, so better to add 'struct vm_area_struct *vma =
vmf->vma;' and
use vma directly
+
+ /* read or shared fault will not split huge pmd */
+ if (!(vmf->flags & FAULT_FLAG_WRITE)
+ || (vmf->vma->vm_flags & VM_SHARED))
+ return VM_FAULT_FALLBACK;
+ if (pe_size != PE_SIZE_PMD)
+ return VM_FAULT_FALLBACK;
return ret；
+
+ if (pmd_none(*vmf->pmd)) {
+ if (shmem_fault(vmf) & VM_FAULT_ERROR)
+ goto out;
+ if (!PageTransHuge(vmf->page))
+ goto out;
+ old_page = vmf->page;
+ } else {
+ old_page = pmd_page(*vmf->pmd);
+ page_remove_rmap(old_page, vmf->vma, true);
+ pmdp_huge_clear_flush(vmf->vma, haddr, vmf->pmd);
+ add_mm_counter(vmf->vma->vm_mm, MM_SHMEMPAGES, -HPAGE_PMD_NR);
MM_SHMEMPAGES -> mm_counter_file(page)
+ }
+
directly use GFP_TRANSHUGE_LIGHT？
+ new_page = &vma_alloc_folio(gfp_flags, HPAGE_PMD_ORDER,
+ vmf->vma, haddr, true)->page;
+ if (!new_page)
add count_vm_event(THP_FAULT_FALLBACK);
+ goto out;
+ prep_transhuge_page(new_page);
vma_alloc_folio() does the prep_transhuge_page() for you.
+ copy_user_huge_page(new_page, old_page, haddr, vmf->vma, HPAGE_PMD_NR);
+ __SetPageUptodate(new_page);
+
+ ret = do_set_pmd(vmf, new_page);
+
+out:
+ if (vmf->page) {
+ unlock_page(vmf->page);
+ put_page(vmf->page);
+ }
+ return ret;
+}
+
unsigned long shmem_get_unmapped_area(struct file *file,
unsigned long uaddr, unsigned long len,
unsigned long pgoff, unsigned long flags)
@@ -3884,6 +3929,7 @@ static const struct super_operations shmem_ops = {
static const struct vm_operations_struct shmem_vm_ops = {
.fault = shmem_fault,
+ .huge_fault = shmem_huge_fault,
.map_pages = filemap_map_pages,
#ifdef CONFIG_NUMA
.set_policy = shmem_set_policy,
--
2.33.0
.
