> On Aug 31, 2023, at 06:47, Mike Kravetz <mike.kravetz@xxxxxxxxxx> wrote:
>
> On 08/30/23 15:26, Muchun Song wrote:
>>
>>
>> On 2023/8/26 03:04, Mike Kravetz wrote:
>>> At the beginning of hugetlb_vmemmap_optimize, optimistically set
>>> the HPageVmemmapOptimized flag in the head page. Clear the flag
>>> if the operation fails.
>>>
>>> No change in behavior. However, this will become important in
>>> subsequent patches where we batch delay TLB flushing. We need to
>>> make sure the content in the old and new vmemmap pages are the same.
>>
>> Sorry, I didn't get the point here. Could you elaborate it?
>>
>
> Sorry, this really could use a better explanation.
>
>>>
>>> Signed-off-by: Mike Kravetz <mike.kravetz@xxxxxxxxxx>
>>> ---
>>> mm/hugetlb_vmemmap.c | 8 +++++---
>>> 1 file changed, 5 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/mm/hugetlb_vmemmap.c b/mm/hugetlb_vmemmap.c
>>> index e390170c0887..500a118915ff 100644
>>> --- a/mm/hugetlb_vmemmap.c
>>> +++ b/mm/hugetlb_vmemmap.c
>>> @@ -566,7 +566,9 @@ static void __hugetlb_vmemmap_optimize(const struct hstate *h,
>>> if (!vmemmap_should_optimize(h, head))
>>> return;
>>> + /* Optimistically assume success */
>>> static_branch_inc(&hugetlb_optimize_vmemmap_key);
>>> + SetHPageVmemmapOptimized(head);
>>> vmemmap_end = vmemmap_start + hugetlb_vmemmap_size(h);
>>> vmemmap_reuse = vmemmap_start;
>>> @@ -577,10 +579,10 @@ static void __hugetlb_vmemmap_optimize(const struct hstate *h,
>>> * to the page which @vmemmap_reuse is mapped to, then free the pages
>>> * which the range [@vmemmap_start, @vmemmap_end] is mapped to.
>>> */
>>> - if (vmemmap_remap_free(vmemmap_start, vmemmap_end, vmemmap_reuse, bulk_pages))
>>> + if (vmemmap_remap_free(vmemmap_start, vmemmap_end, vmemmap_reuse, bulk_pages)) {
>>> static_branch_dec(&hugetlb_optimize_vmemmap_key);
>>> - else
>>> - SetHPageVmemmapOptimized(head);
>>> + ClearHPageVmemmapOptimized(head);
>>> + }
>
> Consider the case where we have successfully remapped vmemmap AND
> - we have replaced the page table page (pte page) containing the struct
> page of the hugetlb head page. Joao's commit 11aad2631bf7
> 'mm/hugetlb_vmemmap: remap head page to newly allocated page'.
> - we have NOT flushed the TLB after remapping due to batching the
> operations before flush.
>
> In this case, it is possible that the old head page is still in the TLB
> and caches and SetHPageVmemmapOptimized(head) will actually set the flag
> in the old pte page. We then have an optimized hugetlb page without the
> HPageVmemmapOptimized flag set. When developing this series, we
> experienced various BUGs as a result of this situation.
Now, I got it. Thanks for your elaboration.
>
> In the case of an error during optimization, we do a TLB flush so if
> we need to clear the flag we will write to the correct pte page.
Right.
>
> Hope that makes sense.
>
> I add an explanation like this to the commit message and perhaps put
> this closer to/or squash with the patch that batches operations before
> flushing TLB.
Yes. But I'd also like to add a big comment to explain what's going on here
instead of a simple "Optimistically assume success". This one really makes
me think it is an optimization not a mandatory premise.
Thanks.
> --
> Mike Kravetz
>
>>> }
>>> /**
