linux-kernel - Re: [PATCH v2 1/3] hugetlb: Convert hugetlb_fault() to use struct vm

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Date: Sun, 7 Apr 2024 15:36:39 +0800
From: Muchun Song <muchun.song@...ux.dev>
To: Vishal Moola <vishal.moola@...il.com>
Cc: Oscar Salvador <osalvador@...e.de>,
 Linux-MM <linux-mm@...ck.org>,
 LKML <linux-kernel@...r.kernel.org>,
 Andrew Morton <akpm@...ux-foundation.org>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>
Subject: Re: [PATCH v2 1/3] hugetlb: Convert hugetlb_fault() to use struct
 vm_fault



> On Apr 5, 2024, at 03:32, Vishal Moola <vishal.moola@...il.com> wrote:
> 
> On Thu, Apr 4, 2024 at 5:26 AM Oscar Salvador <osalvador@...e.de> wrote:
>> 
>> On Mon, Apr 01, 2024 at 01:26:49PM -0700, Vishal Moola (Oracle) wrote:
>>> Now that hugetlb_fault() has a vm_fault available for fault tracking, use
>>> it throughout. This cleans up the code by removing 2 variables, and
>>> prepares hugetlb_fault() to take in a struct vm_fault argument.
>>> 
>>> Signed-off-by: Vishal Moola (Oracle) <vishal.moola@...il.com>
>> 
>> Reviewed-by: Oscar Salvador <osalvador@...e.de>
>> 
>> A question below:
>> 
>>> mm/hugetlb.c | 84 +++++++++++++++++++++++++---------------------------
>>> 1 file changed, 41 insertions(+), 43 deletions(-)
>>> 
>>> diff --git a/mm/hugetlb.c b/mm/hugetlb.c
>>> index 8267e221ca5d..360b82374a89 100644
>>> --- a/mm/hugetlb.c
>>> +++ b/mm/hugetlb.c
>> ...
>>>      /*
>>> -      * entry could be a migration/hwpoison entry at this point, so this
>>> -      * check prevents the kernel from going below assuming that we have
>>> -      * an active hugepage in pagecache. This goto expects the 2nd page
>>> -      * fault, and is_hugetlb_entry_(migration|hwpoisoned) check will
>>> -      * properly handle it.
>>> +      * vmf.orig_pte could be a migration/hwpoison vmf.orig_pte at this
>> 
>> "vmf.orig_pte could be a migration/hwpoison entry at ..."
>> 
>>> -     entry = pte_mkyoung(entry);
>>> -     if (huge_ptep_set_access_flags(vma, haddr, ptep, entry,
>>> +     vmf.orig_pte = pte_mkyoung(vmf.orig_pte);
>>> +     if (huge_ptep_set_access_flags(vma, vmf.address, vmf.pte, vmf.orig_pte,
>>>                                              flags & FAULT_FLAG_WRITE))
>> 
>> Would it make sense to teach huge_ptep_set_access_flags/set_huge_pte_at() to use
>> vm_fault struct as well? All info we are passing is stored there.
>> Maybe it is not worth the trouble though, just asking.
> 
> Yeah, it makes sense. There are actually many function calls in the
> hugetlb_fault() and
> __handle_mm_fault() pathways that could make use of vm_fault to clean
> up the stack.
> 
> It's not particularly complicated either, aside from reorganizing some
> variables for every
> implementation of each function. I'm not really sure if it's worth
> dedicated effort
> and churn though (at least I'm not focused on that for now).

Not all the users of set_huge_pte_at() have a vmf structure. So I do not
think it is a good idea to change it. And huge_ptep_set_access_flags() is
a variant of ptep_set_access_flags(), it's better to keep consistent.
Otherwise, I think both of them should be adapted if you want cleanup.
My tendency is to remain unchanged.

Muchun,
Thanks.