On 2024/11/18 17:29, David Hildenbrand wrote:
On 18.11.24 04:35, Qi Zheng wrote:
On 2024/11/15 22:59, David Hildenbrand wrote:
On 15.11.24 15:41, Qi Zheng wrote:
On 2024/11/15 18:22, David Hildenbrand wrote:
*nr_skip = nr;
and then:
zap_pte_range
--> nr = do_zap_pte_range(tlb, vma, pte, addr, end, details,
&skip_nr,
rss, &force_flush, &force_break);
if (can_reclaim_pt) {
none_nr += count_pte_none(pte, nr);
none_nr += nr_skip;
}
Right?
Yes. I did not look closely at the patch that adds the counting of
Got it.
pte_none though (to digest why it is required :) ).
Because 'none_nr == PTRS_PER_PTE' is used in patch #7 to detect
empty PTE page.
Okay, so the problem is that "nr" would be "all processed entries" but
there are cases where we "process an entry but not zap it".
What you really only want to know is "was any entry not zapped", which
could be a simple input boolean variable passed into do_zap_pte_range?
Because as soon as any entry was processed but no zapped, you can
immediately give up on reclaiming that table.
Yes, we can set can_reclaim_pt to false when a !pte_none() entry is
found in count_pte_none().
I'm not sure if well need cont_pte_none(), but I'll have to take a look
at your new patch to see how this fits together with doing the pte_none
detection+skipping in do_zap_pte_range().
I was wondering if you cannot simply avoid the additional scanning and
simply set "can_reclaim_pt" if you skip a zap.
Maybe we can return the information whether the zap was skipped from
zap_present_ptes() and zap_nonpresent_ptes() through parameters like I
did in [PATCH v1 3/7] and [PATCH v1 4/7].
In theory, we can detect empty PTE pages in the following two ways:
1) If no zap is skipped, it means that all pte entries have been
zap, and the PTE page must be empty.
2) If all pte entries are detected to be none, then the PTE page is
empty.
In the error case, 1) may cause non-empty PTE pages to be reclaimed
(which is unacceptable), while the 2) will at most cause empty PTE pages
to not be reclaimed.
So the most reliable and efficient method may be:
a. If there is a zap that is skipped, stop scanning and do not reclaim
the PTE page;
b. Otherwise, as now, detect the empty PTE page through count_pte_none()
Is there a need for count_pte_none() that I am missing?
When any_skipped == false, at least add VM_BUG_ON() to recheck none ptes.
Assume we have
nr = do_zap_pte_range(&any_skipped)
If "nr" is the number of processed entries (including pte_none()), and
"any_skipped" is set whenever we skipped to zap a !pte_none entry, we
can detect what we need, no?
If any_skipped == false after the call, we now have "nr" pte_none()
entries. -> We can continue trying to reclaim
I prefer that "nr" should not include pte_none().
Something like this:
nr = do_zap_pte_range(&any_skipped);
if (can_reclaim_pt) {
VM_BUG_ON(!any_skipped && count_pte_none(nr) == nr);
if (any_skipped)
can_reclaim_pt = false;
}
nr: the number of processed entries (excluding pte_none())
any_skipped: set whenever we skipped to zap a !pte_none entry
```
do_zap_pte_range
--> pte_t ptent = ptep_get(pte);
int max_nr = (end - addr) / PAGE_SIZE;
/* Skip all consecutive pte_none(). */
if (pte_none(ptent)) {
int nr;
for (nr = 1; nr < max_nr; nr++) {
ptent = ptep_get(pte + nr);
if (!pte_none(ptent))
break;
}
max_nr -= nr;
if (!max_nr)
return 0;
pte += nr;
addr += nr * PAGE_SIZE;
}
if (pte_present(ptent))
return zap_present_ptes(tlb, vma, pte, ptent, max_nr,
addr, details, rss, force_flush,
force_break, any_skipped);
return zap_nonpresent_ptes(tlb, vma, pte, ptent, max_nr, addr,
details, rss, any_skipped);
```
If any_skipped == true, we have at least one !pte_none() entry among the
"nr" entries. -> We cannot and must not reclaim.
