On 10.01.24 10:20, Chen Haixiang wrote:
Transparent hugepages in tmpfs can enhance TLB efficiency by reducing
TLB misses. However, during Copy-On-Write (COW) memory faults, these
hugepages may be split. In some scenarios, preventing this splitting
is desirable. We might introduce a shmem_huge_fault to inhibit this
behavior, along with a mount parameter to enable or disable this function.
I'm confused, can you describe the problem a bit better, because ...
Signed-off-by: Chen Haixiang <chenhaixiang3@xxxxxxxxxx>
---
[...]
+static vm_fault_t shmem_huge_fault(struct vm_fault *vmf, pmd_t orig_pmd)
+{
+ vm_fault_t ret = VM_FAULT_FALLBACK;
+ unsigned long haddr = vmf->address & HPAGE_PMD_MASK;
+ struct folio *old_folio, *new_folio;
+ pmd_t entry;
+ int gfp_flags = GFP_HIGHUSER_MOVABLE | __GFP_COMP;
+ struct vm_area_struct *vma = vmf->vma;
+ struct shmem_sb_info *sbinfo = NULL;
+ struct inode *inode = file_inode(vma->vm_file);
+ struct shmem_inode_info *info = SHMEM_I(inode);
+
+ sbinfo = SHMEM_SB(info->vfs_inode.i_sb);
+
+ if (sbinfo->no_split == 0)
+ return VM_FAULT_FALLBACK;
+
+ /* ShmemPmdMapped in tmpfs will not split huge pmd */
+ if (!(vmf->flags & FAULT_FLAG_WRITE)
+ || (vma->vm_flags & VM_SHARED))
+ return VM_FAULT_FALLBACK;
We do have a private (COW) mapping at this point, and
+
+ new_folio = vma_alloc_folio(gfp_flags, HPAGE_PMD_ORDER,
+ vmf->vma, haddr, true);
+ if (!new_folio)
+ ret = VM_FAULT_FALLBACK;
+
+ vmf->ptl = pmd_lock(vma->vm_mm, vmf->pmd);
+ if (pmd_none(*vmf->pmd)) {
+ ret = VM_FAULT_FALLBACK;
+ goto out;
+ }
+ if (!pmd_same(*vmf->pmd, orig_pmd)) {
+ ret = 0;
+ goto out;
+ }
+
+ if (!new_folio) {
+ count_vm_event(THP_FAULT_FALLBACK);
+ ret = VM_FAULT_FALLBACK;
+ goto out;
+ }
+ old_folio = page_folio(pmd_page(*vmf->pmd));
+ page_remove_rmap(&old_folio->page, vma, true);
+ pmdp_huge_clear_flush(vma, haddr, vmf->pmd);
+
+ __folio_set_locked(new_folio);
+ __folio_set_swapbacked(new_folio);
+ __folio_mark_uptodate(new_folio);
We allocate a fresh folio here and
+
+ flush_icache_pages(vma, &new_folio->page, HPAGE_PMD_NR);
+ entry = mk_huge_pmd(&new_folio->page, vma->vm_page_prot);
+ entry = maybe_pmd_mkwrite(pmd_mkdirty(entry), vma);
+
+ page_add_file_rmap(&new_folio->page, vma, true);
it is not an anonymous one?
... but your are making it writable? In a private mapping?
+ set_pmd_at(vma->vm_mm, haddr, vmf->pmd, entry);
+ update_mmu_cache_pmd(vma, haddr, vmf->pmd);
+ count_vm_event(THP_FILE_MAPPED);
And still acoount it as a file folio? But never add it so some pagecache
structure?
I'm probably missing something, or something is completely wrong here.
--
Cheers,
David / dhildenb
