On 08/18/2020 05:56 PM, Jonathan Cameron wrote: > On Tue, 18 Aug 2020 15:11:58 +0530 > Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote: > >> On 08/18/2020 02:43 PM, Jonathan Cameron wrote: >>> On Mon, 17 Aug 2020 14:49:43 +0530 >>> Anshuman Khandual <anshuman.khandual@xxxxxxx> wrote: >>> >>>> pmd_present() and pmd_trans_huge() are expected to behave in the following >>>> manner during various phases of a given PMD. It is derived from a previous >>>> detailed discussion on this topic [1] and present THP documentation [2]. >>>> >>>> pmd_present(pmd): >>>> >>>> - Returns true if pmd refers to system RAM with a valid pmd_page(pmd) >>>> - Returns false if pmd does not refer to system RAM - Invalid pmd_page(pmd) >>>> >>>> pmd_trans_huge(pmd): >>>> >>>> - Returns true if pmd refers to system RAM and is a trans huge mapping >>>> >>>> ------------------------------------------------------------------------- >>>> | PMD states | pmd_present | pmd_trans_huge | >>>> ------------------------------------------------------------------------- >>>> | Mapped | Yes | Yes | >>>> ------------------------------------------------------------------------- >>>> | Splitting | Yes | Yes | >>>> ------------------------------------------------------------------------- >>>> | Migration/Swap | No | No | >>>> ------------------------------------------------------------------------- >>>> >>>> The problem: >>>> >>>> PMD is first invalidated with pmdp_invalidate() before it's splitting. This >>>> invalidation clears PMD_SECT_VALID as below. >>>> >>>> PMD Split -> pmdp_invalidate() -> pmd_mkinvalid -> Clears PMD_SECT_VALID >>>> >>>> Once PMD_SECT_VALID gets cleared, it results in pmd_present() return false >>>> on the PMD entry. It will need another bit apart from PMD_SECT_VALID to re- >>>> affirm pmd_present() as true during the THP split process. To comply with >>>> above mentioned semantics, pmd_trans_huge() should also check pmd_present() >>>> first before testing presence of an actual transparent huge mapping. >>>> >>>> The solution: >>>> >>>> Ideally PMD_TYPE_SECT should have been used here instead. But it shares the >>>> bit position with PMD_SECT_VALID which is used for THP invalidation. Hence >>>> it will not be there for pmd_present() check after pmdp_invalidate(). >>>> >>>> A new software defined PMD_PRESENT_INVALID (bit 59) can be set on the PMD >>>> entry during invalidation which can help pmd_present() return true and in >>>> recognizing the fact that it still points to memory. >>>> >>>> This bit is transient. During the split process it will be overridden by a >>>> page table page representing normal pages in place of erstwhile huge page. >>>> Other pmdp_invalidate() callers always write a fresh PMD value on the entry >>>> overriding this transient PMD_PRESENT_INVALID bit, which makes it safe. >>>> >>>> [1]: https://lkml.org/lkml/2018/10/17/231 >>>> [2]: https://www.kernel.org/doc/Documentation/vm/transhuge.txt >>> >>> Hi Anshuman, >>> >>> One query on this. From my reading of the ARM ARM, bit 59 is not >>> an ignored bit. The exact requirements for hardware to be using >>> it are a bit complex though. >>> >>> It 'might' be safe to use it for this, but if so can we have a comment >>> explaining why. Also more than possible I'm misunderstanding things! >> >> We are using this bit 59 only when the entry is not active from MMU >> perspective i.e PMD_SECT_VALID is clear. >> > > Understood. I guess we ran out of bits that were always ignored so had > to start using ones that are ignored in this particular state. Right, there are no more available SW PTE bits. #define PTE_DIRTY (_AT(pteval_t, 1) << 55) #define PTE_SPECIAL (_AT(pteval_t, 1) << 56) #define PTE_DEVMAP (_AT(pteval_t, 1) << 57) #define PTE_PROT_NONE (_AT(pteval_t, 1) << 58) /* only when !PTE_VALID */ Earlier I had proposed using PTE_SPECIAL at PMD level for this purpose. But Catalin prefers these unused bits as the entry is anyway invalid and which also leaves aside PTE_SPECIAL at mapped PMD for later use. There is already one comment near PMD_PRESENT_INVALID definition which explains this situation. +/* + * This help indicate that the entry is present i.e pmd_page() + * still points to a valid huge page in memory even if the pmd + * has been invalidated. + */ +#define PMD_PRESENT_INVALID (_AT(pteval_t, 1) << 59) /* only when !PMD_SECT_VALID */
