Re: [PATCH] hugetlb: simplify hugetlb handling in follow_page_mask

[Baolin Wang]

On 9/1/2022 8:00 AM, Mike Kravetz wrote:
> On 08/31/22 09:07, Baolin Wang wrote:
> > On 8/31/2022 2:39 AM, Mike Kravetz wrote:
> > > On 08/30/22 09:44, Mike Kravetz wrote:
> > > > On 08/30/22 09:06, Baolin Wang wrote:
> > > > > Hi Mike,
> > > > >
> > > > > On 8/30/2022 7:40 AM, Mike Kravetz wrote:
> > > > > > During discussions of this series [1], it was suggested that hugetlb
> > > > > > handling code in follow_page_mask could be simplified.  At the beginning
> > > > > > of follow_page_mask, there currently is a call to follow_huge_addr which
> > > > > > 'may' handle hugetlb pages.  ia64 is the only architecture which provides
> > > > > > a follow_huge_addr routine that does not return error.  Instead, at each
> > > > > > level of the page table a check is made for a hugetlb entry.  If a hugetlb
> > > > > > entry is found, a call to a routine associated with that entry is made.
> > > > > >
> > > > > > Currently, there are two checks for hugetlb entries at each page table
> > > > > > level.  The first check is of the form:
> > > > > > 	if (p?d_huge())
> > > > > > 		page = follow_huge_p?d();
> > > > > > the second check is of the form:
> > > > > > 	if (is_hugepd())
> > > > > > 		page = follow_huge_pd().
> > > > > >
> > > > > > We can replace these checks, as well as the special handling routines
> > > > > > such as follow_huge_p?d() and follow_huge_pd() with a single routine to
> > > > > > handle hugetlb vmas.
> > > > > >
> > > > > > A new routine hugetlb_follow_page_mask is called for hugetlb vmas at the
> > > > > > beginning of follow_page_mask.  hugetlb_follow_page_mask will use the
> > > > > > existing routine huge_pte_offset to walk page tables looking for hugetlb
> > > > > > entries.  huge_pte_offset can be overwritten by architectures, and already
> > > > > > handles special cases such as hugepd entries.
> > > > >
> > > > > Could you also mention that this patch will fix the lock issue for
> > > > > CONT-PTE/PMD hugetlb by changing to use huge_pte_lock()? which will help
> > > > > people to understand the issue.
> > > >
> > > > Will update message in v2.  Thanks for taking a look!
> > >
> > > One additional thought, we 'may' need a separate patch to fix the locking
> > > issues that can be easily backported.  Not sure this 'simplification' is
> > > a good backport candidate.
> >
> > Yes, that was my thought before, but David did not like adding more
> > make-legacy-cruft-happy code.
> >
> > So how about creating a series that contains 3 patches: picking up patch 1
> > and patch 3 of my previous series [1], and your current patch? That means
> > patch 1 and patch 2 in this series can fix the lock issue explicitly and be
> > suitable to backport, meanwhile patch 3 (which is your current patch) will
> > cleanup the legacy code.
>
> When I looked at patch 3, I was thinking the update follow_huge_pmd routine
> would work for the PTE level with a few more modifications.  Perhaps, this is
> too ugly but it is a smaller set of changes for backport.
>
> Of course, this would be followed up with the simplification patch which
> removes all this code.

Yes, looks more simple. I can send you a formal patch with your 
suggestion, which can be added into your cleanup series. Thanks.