Re: [f2fs-dev] [PATCH 06/14] f2fs: prepare mmap write faults for large folios

[Nanzhe Zhao]

On Thu, 27 Aug 2026 20:36:02 +0800, Chao Yu wrote:
> We only need to call f2fs_get_block_locked() for vmf->page?
>
> Hi Barry, could you please help to confirm this? Only vmf->page contain dirty
> data, rather than whole large folio contain dirty data?

Thanks for the review. Here is the write-fault path on current
mainline (mm/memory.c):

do_pte_missing()                          memory.c:4561
  -> do_fault()                           memory.c:5964
    -> do_shared_fault()                  memory.c:5914
      -> __do_fault() -> filemap_fault()  : fault in folio (may be 16KB)
      -> do_page_mkwrite(vmf, folio)      : page_mkwrite ONCE per folio (memory.c:5936)
         -> vma->vm_ops->page_mkwrite(vmf)  memory.c:3684
      -> finish_fault()                   memory.c:5617
         -> set_pte_range()               memory.c:5558
            if (write)
                entry = maybe_mkwrite(pte_mkdirty(entry), vma);   memory.c:5575
            set_ptes(vma->vm_mm, addr, vmf->pte, entry, nr);      memory.c:5586

So page_mkwrite is entered once per folio, and afterwards the PTEs of
all subpages are installed writable in one go - writes to other
subpages of the same folio will not fault again. That is why
f2fs_vm_page_mkwrite() preallocates/marks dirty_len rather than
handling vmf->page only.

I'll add a comment in f2fs_vm_page_mkwrite() in the next version to
make the point clear.

Thanks,
Nanzhe