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

[Dev Jain]

On 05/03/25 4:16 pm, 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@xxxxxxxxxxxxx>
> 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).
>
> 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.

Thanks for your reply David.

> > 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.

I had a mental hiccup, yes we don't need the cached value even before 
the if block, as the relevant path will eventually check after taking 
the lock. I was thinking of all sorts of weird races.

> 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)

Oh I just looked at the arm64 definition and assumed ptep_get_lockless() 
== READ_ONCE() :) Now this makes sense. So I am guessing that we can 
still get away with a *vmf.pmd on 64-bit arches?

A separate question: When taking the create_huge_pmd() path with a read 
fault and after taking the pmd lock, why shouldn't we check with 
pmd_none(pmdp_get_lockless(vmf.pmd)) instead of plain *vmf.pmd...surely 
now we must load the actual correct value from memory since we are 
committing into mapping the huge zero folio?
And after looking somewhat more, why even is a pmd_none(*pmd) there in 
set_huge_zero_folio()...it should be the responsibility of the caller to 
verify this? Any caller will just assume that it got the huge zero folio 
mapped so this check should be redundant.

> 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).