Re: [PATCH v2 01/14] mm: pgtable: introduce pte_offset_map_{ro|rw}_nolock()

[Qi Zheng <zhengqi.arch@bytedance.com>]

On 2024/8/28 18:48, David Hildenbrand wrote:
> On 27.08.24 06:33, Qi Zheng wrote:

[...]

>> sufficient AFAIUK.
> 
> Drop the "AFAIUK" :)
> 
> "For R/O access this is sufficient."
> 
>>
>> pte_offset_map_rw_nolock(mm, pmd, addr, pmdvalp, ptlp), above, is like
>> pte_offset_map_ro_nolock(); but when successful, it also outputs the
>> pdmval. For R/W access, the callers can not accept that the page table
>> it sees has been unmapped and is about to get freed. The pmdval can help
>> callers to recheck pmd_same() to identify this case once the spinlock is
>> taken. For some cases where exclusivity is already guaranteed, such as
>> holding the write lock of mmap_lock, or in cases where checking is
>> sufficient, such as a !pte_none() pte will be rechecked after the
>> spinlock is taken, there is no need to recheck pdmval.
> 
> Right, using pte_same() one can achieve a similar result, assuming that 
> the freed page table gets all ptes set to pte_none().
> 
> page_table_check_pte_clear_range() before pte_free_defer() in 
> retract_page_tables/collapse_pte_mapped_thp() sanity checks that I think.

Since commit 1d65b771bc08, retract_page_tables() only holds the
i_mmap_lock_read(mapping) but not mmap_lock, so it seems that
holding the write lock of mmap_lock cannot guarantee the stability
of the PTE page.

IIUC, I will also perform a pmd_same() check on the case where the
write lock of mmap_lock is held in v3. Or do I miss something?

> 
> In collapse_huge_page() that is not the case. But here, we also 
> currently grab all heavily locks, to prevent any concurrent page table 
> walker.
> 
>>
>> Note: "RO" / "RW" expresses the intended semantics, not that the *kmap*
>> will be RO/RW protected.
> 
> 
> Good. Please also incorporate the feedback from Muchun.
>