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Subject: Re: [PATCH v2 1/3] hugetlb: Convert hugetlb_fault() to use struct
 vm_fault
From: Muchun Song <muchun.song@linux.dev>
In-Reply-To: <CAOzc2pxfUSnT6fEXAJZowYeMz=hCgWsfJRB++dis1AL_rNScGQ@mail.gmail.com>
Date: Sun, 7 Apr 2024 15:36:39 +0800
Cc: Oscar Salvador <osalvador@suse.de>,
 Linux-MM <linux-mm@kvack.org>,
 LKML <linux-kernel@vger.kernel.org>,
 Andrew Morton <akpm@linux-foundation.org>,
 "Matthew Wilcox (Oracle)" <willy@infradead.org>
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Message-Id: <EE21834D-E805-40AE-AFC3-E43C72BE5A15@linux.dev>
References: <20240401202651.31440-1-vishal.moola@gmail.com>
 <20240401202651.31440-2-vishal.moola@gmail.com>
 <Zg6cualUqcsZYZxf@localhost.localdomain>
 <CAOzc2pxfUSnT6fEXAJZowYeMz=hCgWsfJRB++dis1AL_rNScGQ@mail.gmail.com>
To: Vishal Moola <vishal.moola@gmail.com>


> On Apr 5, 2024, at 03:32, Vishal Moola <vishal.moola@gmail.com> wrote:
>=20
> On Thu, Apr 4, 2024 at 5:26=E2=80=AFAM Oscar Salvador =
<osalvador@suse.de> wrote:
>>=20
>> 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.
>>>=20
>>> Signed-off-by: Vishal Moola (Oracle) <vishal.moola@gmail.com>
>>=20
>> Reviewed-by: Oscar Salvador <osalvador@suse.de>
>>=20
>> A question below:
>>=20
>>> mm/hugetlb.c | 84 =
+++++++++++++++++++++++++---------------------------
>>> 1 file changed, 41 insertions(+), 43 deletions(-)
>>>=20
>>> 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
>>=20
>> "vmf.orig_pte could be a migration/hwpoison entry at ..."
>>=20
>>> -     entry =3D pte_mkyoung(entry);
>>> -     if (huge_ptep_set_access_flags(vma, haddr, ptep, entry,
>>> +     vmf.orig_pte =3D pte_mkyoung(vmf.orig_pte);
>>> +     if (huge_ptep_set_access_flags(vma, vmf.address, vmf.pte, =
vmf.orig_pte,
>>>                                              flags & =
FAULT_FLAG_WRITE))
>>=20
>> 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.
>=20
> 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.
>=20
> 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.