On 5/8/2022 11:26 PM, Muchun Song wrote:
On Sun, May 08, 2022 at 04:58:51PM +0800, Baolin Wang wrote:
Hi,
As Mike pointed out [1], the huge_ptep_get() will only return one specific
pte value for the CONT-PTE or CONT-PMD size hugetlb on ARM64 system, which
will not take into account the subpages' dirty or young bits of a CONT-PTE/PMD
size hugetlb page. That will make us miss dirty or young flags of a CONT-PTE/PMD
size hugetlb page for those functions that want to check the dirty or
young flags of a hugetlb page. For example, the gather_hugetlb_stats() will
get inaccurate dirty hugetlb page statistics, and the DAMON for hugetlb monitoring
will also get inaccurate access statistics.
To fix this issue, one approach is that we can define an ARM64 specific huge_ptep_get()
implementation, which will take into account any subpages' dirty or young bits.
IIUC, we could get the page size by page_size(pte_page(pte)).
So, how about the following implementation of huge_ptep_get()?
Does this work for you?
pte_t huge_ptep_get(pte_t *ptep)
{
int ncontig, i;
size_t pgsize;
pte_t orig_pte = ptep_get(ptep);
if (!pte_present(orig_pte) || !pte_cont(orig_pte))
return orig_pte;
ncontig = num_contig_ptes(page_size(pte_page(orig_pte)), &pgsize);
for (i = 0; i < ncontig; i++, ptep++) {
pte_t pte = ptep_get(ptep);
if (pte_dirty(pte))
orig_pte = pte_mkdirty(orig_pte);
if (pte_young(pte))
orig_pte = pte_mkyoung(orig_pte);
}
return orig_pte;
}
Thanks for your suggestion, and I think this works for me and looks more
straight forward in case some functions using huge_ptep_get() will care
about the young or dirty bits in future.
My only concern is that all the functions using huge_ptep_get() will set
a contPTE dirty or accessed bit, however most functions do not care
about the dirty and accessed bit, which becomes a bit more expensive for
them? Also mentioned by Matthew in his comments. Anyway, I still think
your suggestion is straight forward and I can change in next version if
no other objections.
