On 06/27/2019 09:01 PM, Zi Yan wrote: > 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/ Sure will update the link in the commit. Thanks !
