Re: [PATCH v6 01/27] mm: Introduce struct folio

Matthew Wilcox
2021-04-06T12:48:52Z

On Tue, Apr 06, 2021 at 03:29:18PM +0300, Kirill A. Shutemov wrote:
> On Wed, Mar 31, 2021 at 07:47:02PM +0100, Matthew Wilcox (Oracle) wrote:
> > +/**
> > + * folio_next - Move to the next physical folio.
> > + * @folio: The folio we're currently operating on.
> > + *
> > + * If you have physically contiguous memory which may span more than
> > + * one folio (eg a &struct bio_vec), use this function to move from one
> > + * folio to the next. Do not use it if the memory is only virtually
> > + * contiguous as the folios are almost certainly not adjacent to each
> > + * other. This is the folio equivalent to writing ``page++``.
> > + *
> > + * Context: We assume that the folios are refcounted and/or locked at a
> > + * higher level and do not adjust the reference counts.
> > + * Return: The next struct folio.
> > + */
> > +static inline struct folio *folio_next(struct folio *folio)
> > +{
> > +#if defined(CONFIG_SPARSEMEM) && !defined(CONFIG_SPARSEMEM_VMEMMAP)
> > + return (struct folio *)nth_page(&folio->page, folio_nr_pages(folio));
> > +#else
> > + return folio + folio_nr_pages(folio);
> > +#endif
>
> Do we really need the #if here?
>
> >From quick look at nth_page() and memory_model.h, compiler should be able
> to simplify calculation for FLATMEM or SPARSEMEM_VMEMMAP to what you do in
> the #else. No?

No.

0000000000001180 :
struct page *a(struct page *p, unsigned long n)
{
1180: e8 00 00 00 00 callq 1185 <a+0x5>
1181: R_X86_64_PLT32 __fentry__-0x4
1185: 55 push %rbp
return nth_page(p, n);
1186: 48 2b 3d 00 00 00 00 sub 0x0(%rip),%rdi
1189: R_X86_64_PC32 vmemmap_base-0x4
118d: 48 c1 ff 06 sar $0x6,%rdi
1191: 48 8d 04 37 lea (%rdi,%rsi,1),%rax
1195: 48 89 e5 mov %rsp,%rbp
return nth_page(p, n);
1198: 48 c1 e0 06 shl $0x6,%rax
119c: 48 03 05 00 00 00 00 add 0x0(%rip),%rax
119f: R_X86_64_PC32 vmemmap_base-0x4
11a3: 5d pop %rbp
11a4: c3 retq

vs

00000000000011b0 :

struct page *b(struct page *p, unsigned long n)
{
11b0: e8 00 00 00 00 callq 11b5 <b+0x5>
11b1: R_X86_64_PLT32 __fentry__-0x4
11b5: 55 push %rbp
return p + n;
11b6: 48 c1 e6 06 shl $0x6,%rsi
11ba: 48 8d 04 37 lea (%rdi,%rsi,1),%rax
11be: 48 89 e5 mov %rsp,%rbp
11c1: 5d pop %rbp
11c2: c3 retq

Now, maybe we should put this optimisation into the definition of nth_page?

> > +struct folio {
> > + /* private: don't document the anon union */
> > + union {
> > + struct {
> > + /* public: */
> > + unsigned long flags;
> > + struct list_head lru;
> > + struct address_space *mapping;
> > + pgoff_t index;
> > + unsigned long private;
> > + atomic_t _mapcount;
> > + atomic_t _refcount;
> > +#ifdef CONFIG_MEMCG
> > + unsigned long memcg_data;
> > +#endif
>
> As Christoph, I'm not a fan of this :/

What would you prefer?