On 23.01.24 20:43, Ryan Roberts wrote:
On 23/01/2024 19:33, David Hildenbrand wrote:
On 23.01.24 20:15, Ryan Roberts wrote:
On 22/01/2024 19:41, David Hildenbrand wrote:
Now that the rmap overhaul[1] is upstream that provides a clean interface
for rmap batching, let's implement PTE batching during fork when processing
PTE-mapped THPs.
This series is partially based on Ryan's previous work[2] to implement
cont-pte support on arm64, but its a complete rewrite based on [1] to
optimize all architectures independent of any such PTE bits, and to
use the new rmap batching functions that simplify the code and prepare
for further rmap accounting changes.
We collect consecutive PTEs that map consecutive pages of the same large
folio, making sure that the other PTE bits are compatible, and (a) adjust
the refcount only once per batch, (b) call rmap handling functions only
once per batch and (c) perform batch PTE setting/updates.
While this series should be beneficial for adding cont-pte support on
ARM64[2], it's one of the requirements for maintaining a total mapcount[3]
for large folios with minimal added overhead and further changes[4] that
build up on top of the total mapcount.
I'm currently rebasing my contpte work onto this series, and have hit a problem.
I need to expose the "size" of a pte (pte_size()) and skip forward to the start
of the next (cont)pte every time through the folio_pte_batch() loop. But
pte_next_pfn() only allows advancing by 1 pfn; I need to advance by nr pfns:
static inline int folio_pte_batch(struct folio *folio, unsigned long addr,
pte_t *start_ptep, pte_t pte, int max_nr, bool *any_writable)
{
unsigned long folio_end_pfn = folio_pfn(folio) + folio_nr_pages(folio);
const pte_t *end_ptep = start_ptep + max_nr;
pte_t expected_pte = __pte_batch_clear_ignored(pte_next_pfn(pte));
- pte_t *ptep = start_ptep + 1;
+ pte_t *ptep = start_ptep;
+ int vfn, nr, i;
bool writable;
if (any_writable)
*any_writable = false;
VM_WARN_ON_FOLIO(!pte_present(pte), folio);
+ vfn = addr >> PAGE_SIZE;
+ nr = pte_size(pte);
+ nr = ALIGN_DOWN(vfn + nr, nr) - vfn;
+ ptep += nr;
+
while (ptep != end_ptep) {
+ pte = ptep_get(ptep);
nr = pte_size(pte);
if (any_writable)
writable = !!pte_write(pte);
pte = __pte_batch_clear_ignored(pte);
if (!pte_same(pte, expected_pte))
break;
/*
* Stop immediately once we reached the end of the folio. In
* corner cases the next PFN might fall into a different
* folio.
*/
- if (pte_pfn(pte) == folio_end_pfn)
+ if (pte_pfn(pte) >= folio_end_pfn)
break;
if (any_writable)
*any_writable |= writable;
- expected_pte = pte_next_pfn(expected_pte);
- ptep++;
+ for (i = 0; i < nr; i++)
+ expected_pte = pte_next_pfn(expected_pte);
+ ptep += nr;
}
return ptep - start_ptep;
}
So I'm wondering if instead of enabling pte_next_pfn() for all the arches,
perhaps its actually better to expose pte_pgprot() for all the arches. Then we
can be much more flexible about generating ptes with pfn_pte(pfn, pgprot).
What do you think?
The pte_pgprot() stuff is just nasty IMHO.
I dunno; we have pfn_pte() which takes a pfn and a pgprot. It seems reasonable
that we should be able to do the reverse.
But pte_pgprot() is only available on a handful of architectures, no? It
would be nice to have a completely generic pte_next_pfn() /
pte_advance_pfns(), though.
Anyhow, this is all "easy" to rework later. Unless I am missing
something, the low hanging fruit is simply using PFN_PTE_SHIFT for now
that exists on most archs already.
Likely it's best to simply convert pte_next_pfn() to something like
pte_advance_pfns(). The we could just have
#define pte_next_pfn(pte) pte_advance_pfns(pte, 1)
That should be fairly easy to do on top (based on PFN_PTE_SHIFT). And only 3
archs (x86-64, arm64, and powerpc) need slight care to replace a hardcoded "1"
by an integer we pass in.
I thought we agreed powerpc was safe to just define PFN_PTE_SHIFT? But, yeah,
the principle works I guess. I guess I can do this change along with my series.
It is, if nobody insists on that micro-optimization on powerpc.
If there is good reason to invest more time and effort right now on the
pte_pgprot approach, then please let me know :)
--
Cheers,
David / dhildenb
