On 27 Jun 2019, at 8:48, Anshuman Khandual 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_mknotpresent -> 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(). > > PTE_SPECIAL never gets used for PMD mapping i.e there is no pmd_special(). > Hence this bit can be set on the PMD entry during invalidation which can > help in making pmd_present() return true and in recognizing the fact that > it still points to memory. > > This bit is transient. During the split is process it will be overridden > by a page table page representing the normal pages in place of erstwhile > huge page. Other pmdp_invalidate() callers always write a fresh PMD value > on the entry overriding this transient PTE_SPECIAL making it safe. In the > past former pmd_[mk]splitting() functions used PTE_SPECIAL. > > [1]: https://lkml.org/lkml/2018/10/17/231 Just want to point out that lkml.org link might not be stable. This one would be better: https://lore.kernel.org/linux-mm/20181017020930.GN30832@xxxxxxxxxx/ — Best Regards, Yan Zi
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