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Howlett" , Julia Lawall , Nicolas Palix , Anshuman Khandual References: <20251113014656.2605447-1-samuel.holland@sifive.com> <20251113014656.2605447-7-samuel.holland@sifive.com> <02e3b3bd-ae6a-4db4-b4a1-8cbc1bc0a1c8@arm.com> Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: 016EE20002 X-Stat-Signature: z1jtxfn6jia3hyqbhn1w7fwzx9km1ysi X-Rspam-User: X-HE-Tag: 1764159547-54422 X-HE-Meta: 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 3BUVAq04 sxcC/56waYtV293lvm+8WSNTXGrYf2ZOM2zteG10lpYbMdOhh/Tjo5KRA+MBc0TQ3OlcyYGwqf+QFkfXRSbjLjSuf70+X3tzHOAzmBYUh+bRWiqiWAtCc6TONfWemk7K8d5pB9PZ7AEbZxg0pMOcugOVQSzY+9VOFGRtZfMCbcQnNdEfJeCu6khMwhRd+sBnA9PpTQKTKPe6h3oZAKreT043VKreXFSdqoFyyVbGo9ORd50bgwcQk3MvHCkdR8KTeSC1Af2AyiIIfzn8c1oDyQUcmSfYegOiJOPzqGNpJOP2aLZF0jL3ZthzGra/EAokrcSVvfcZyvsRuaiYSiR12sDofVNDIxNIa31s0SDKKXTmG1O6oyoq9O/hXc/1BXt92wvxDctTN5EUTUC+RWPE/f+BQsY5fUpDxS2eOKkpJq9enPR1RC/MqxH5WLg== X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 11/26/25 13:16, David Hildenbrand (Red Hat) wrote: > On 11/26/25 12:09, Ryan Roberts wrote: >> On 13/11/2025 01:45, Samuel Holland wrote: >>> Some platforms need to fix up the values when reading or writing page >>> tables. Because of this, the accessors must always be used; it is not >>> valid to simply dereference a pXX_t pointer. >>> >>> Fix all of the instances of this pattern in generic code, mostly by >>> applying the below coccinelle semantic patch, repeated for each page >>> table level. Some additional fixes were applied manually, mostly to >>> macros where type information is unavailable. >>> >>> In a few places, a `pte_t *` or `pmd_t *` is actually a pointer to a PTE >>> or PMDE value stored on the stack, not a pointer to a page table. In >>> those cases, it is not appropriate to use the accessors, because the >>> value is not globally visible, and any transformation from pXXp_get() >>> has already been applied. Those places are marked by naming the pointer >>> `ptentp` or `pmdvalp`, as opposed to `ptep` or `pmdp`. >>> >>> @@ >>> pte_t *P; >>> expression E; >>> expression I; >>> @@ >>> - P[I] = E >>> + set_pte(P + I, E) >>> >>> @@ >>> pte_t *P; >>> expression E; >>> @@ >>> ( >>> - WRITE_ONCE(*P, E) >>> + set_pte(P, E) >>> | >>> - *P = E >>> + set_pte(P, E) >>> ) >> >> There should absolutely never be any instances of core code directly setting an >> entry at any level. This *must* always go via the arch code helpers. Did you >> find any instances of this? If so, I would consider these bugs and suggest >> sending as a separate bugfix patch. Bad things could happen on arm64 because we >> may need to break a contiguous mapping, which would not happen if the value is >> set directly. >> >>> >>> @@ >>> pte_t *P; >>> expression I; >>> @@ >>> ( >>> &P[I] >>> | >>> - READ_ONCE(P[I]) >>> + ptep_get(P + I) >>> | >>> - P[I] >>> + ptep_get(P + I) >>> ) >>> >>> @@ >>> pte_t *P; >>> @@ >>> ( >>> - READ_ONCE(*P) >>> + ptep_get(P) >>> | >>> - *P >>> + ptep_get(P) >>> ) >> >> For reading the *PTE*, conversion over to ptep_get() should have already been >> done (I did this a few years back when implementing support for arm64 contiguous >> mappings). If you find any cases where direct dereference or READ_ONCE() is >> being done in generic code for PTE, then that's a bug and should also be sent as >> a separate patch. >> >> FYI, my experience was that Coccinelle didn't find everything when I was >> converting to ptep_get() - although it could have been that my Cochinelle skills >> were not up to scratch! I ended up using an additional method where I did a >> find/replace to convert "pte_t *" to "ptep_handle_t" and declared pte_handle_t >> as a void* which causes a compiler error on dereference. Then in a few key >> places I did a manual case from pte_handle_t to (pte_t *) and compiled allyesconfig. >> >> I'm assuming the above Cocchinelle template was also used for pmd_t, pud_t, >> p4d_t and pgd_t? >> >>> >>> Additionally, the following semantic patch was used to convert PMD and >>> PUD references inside struct vm_fault: >>> >>> @@ >>> struct vm_fault vmf; >>> @@ >>> ( >>> - *vmf.pmd >>> + pmdp_get(vmf.pmd) >>> | >>> - *vmf.pud >>> + pudp_get(vmf.pud) >>> ) >>> >>> @@ >>> struct vm_fault *vmf; >>> @@ >>> ( >>> - *vmf->pmd >>> + pmdp_get(vmf->pmd) >>> | >>> - *vmf->pud >>> + pudp_get(vmf->pud) >>> ) >>> >>> >>> Signed-off-by: Samuel Holland >>> --- >>> This commit covers some of the same changes as an existing series from >>> Anshuman Khandual[1]. Unlike that series, this commit is a purely >>> mechanical conversion to demonstrate the RISC-V changes, so it does not >>> insert local variables to avoid redundant calls to the accessors. A >>> manual conversion like in that series could improve performance. >>> >>> [1]: https://lore.kernel.org/linux-mm/20240917073117.1531207-1-anshuman.khandual@arm.com/ >> >> Hi, >> >> I've just come across this patch and wanted to mention that we could also >> benefit from this improved absraction for some features we are looking at for >> arm64. As you mention, Anshuman had a go but hit some roadblocks. >> >> The main issue is that the compiler was unable to optimize away the READ_ONCE()s >> for the case where certain levels of the pgtable are folded. But it can optimize >> the plain C dereferences. There were complaints the the generated code for arm >> (32) and powerpc was significantly impacted due to having many more (redundant) >> loads. >> > > We do have mm_pmd_folded()/p4d_folded() etc, could that help to sort > this out internally? > Just stumbled over the reply from Christope: https://lkml.kernel.org/r/0019d675-ce3d-4a5c-89ed-f126c45145c9@kernel.org And wonder if we could handle that somehow directly in the pgdp_get() etc. -- Cheers David