Re: [QUESTION] Plain dereference and READ_ONCE() in fault handler

[Matthew Wilcox <willy@infradead.org>]

On Wed, Mar 05, 2025 at 11:46:41AM +0100, David Hildenbrand wrote:
> On 05.03.25 11:21, Dev Jain wrote:
> > In __handle_mm_fault(),
> > 
> > 1. Why is there a barrier() for the PUD logic?
> 
> Good question. It was added in
> 
> commit a00cc7d9dd93d66a3fb83fc52aa57a4bec51c517
> Author: Matthew Wilcox <willy@infradead.org>
> Date:   Fri Feb 24 14:57:02 2017 -0800
> 
>     mm, x86: add support for PUD-sized transparent hugepages
> 
> Maybe it was an alternative to performing a READ_ONCE(*vmf.pud).

I was monkey-see, monkey-do.

Here's the corresponding code as it existed at the time:

        } else {
                pmd_t orig_pmd = *vmf.pmd;

                barrier();
                if (pmd_trans_huge(orig_pmd) || pmd_devmap(orig_pmd)) {
                        vmf.flags |= FAULT_FLAG_SIZE_PMD;

vs what I added:

        } else {
                pud_t orig_pud = *vmf.pud;

                barrier();
                if (pud_trans_huge(orig_pud) || pud_devmap(orig_pud)) {

At some point, somebody added pmdp_get_lockless() and did not add a
corresponding pudp_get_lockless().  And it was ... Hugh in 26e1a0c3277d

If you want to add a pudp_get_lockless(), I doubt anyone will object,
but it's probably pointless churn.

> Maybe it was done that way, because pudp_get_lockless() does not exist. And
> it would likely not be required, because on architectures where
> ptep_get_lockless() does some magic (see below, mostly 32bit), PUD THP are
> not applicable.
> 
> 
> > 2. For the PMD logic, in the if block, we use *vmf.pmd, and in the else block
> >     we use pmdp_get_lockless(); what if someone changes the pmd just when we
> >     have begun processing the conditions in the if block, fail in the if block
> >     and then the else block operates on a different pmd value. Shouldn't we cache
> >     the value of the pmd and operate on a single consistent value until we take the
> >     lock and then finally check using pxd_same() and friends?
> 
> The pmd_none(*vmf.pmd) is fine. create_huge_pmd() must be able to deal with
> races, because we are not holding any locks.
> 
> We only have to use pmdp_get_lockless() when we want to effectively perform
> a READ_ONCE(), and make sure that we read something "reasonable" that we can
> operate on, even with concurrent changes. (e.g., not read a garbage PFN just
> because of some concurrent changes)
> 
> We'll store the value in vmf.orig_pmd, on which we'll operate and try to
> detect later changes using pmd_same(). So we really want something
> consistent in there.
> 
> See the description above ptep_get_lockless(), why we cannot simply do a
> READ_ONCE on architectures where a PTE cannot be read atomically (e.g., 8
> byte PTEs on 32bit architecture).
> 
> -- 
> Cheers,
> 
> David / dhildenb
>