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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> From: "David Hildenbrand (Red Hat)" Content-Language: en-US In-Reply-To: <02e3b3bd-ae6a-4db4-b4a1-8cbc1bc0a1c8@arm.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam03 X-Rspamd-Queue-Id: B9FBD100015 X-Stat-Signature: w33jxx59m1xxzhzjxjxh1d44oxz7q8n9 X-Rspam-User: X-HE-Tag: 1764159403-182846 X-HE-Meta: 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 bsA== 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 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? -- Cheers David