On Tuesday, 9 March 2021 5:58:12 AM AEDT Ralph Campbell wrote: > > On 3/3/21 10:16 PM, Alistair Popple wrote: > > Migration is currently implemented as a mode of operation for > > try_to_unmap_one() generally specified by passing the TTU_MIGRATION flag > > or in the case of splitting a huge anonymous page TTU_SPLIT_FREEZE. > > > > However it does not have much in common with the rest of the unmap > > functionality of try_to_unmap_one() and thus splitting it into a > > separate function reduces the complexity of try_to_unmap_one() making it > > more readable. > > > > Several simplifications can also be made in try_to_migrate_one() based > > on the following observations: > > > > - All users of TTU_MIGRATION also set TTU_IGNORE_MLOCK. > > - No users of TTU_MIGRATION ever set TTU_IGNORE_HWPOISON. > > - No users of TTU_MIGRATION ever set TTU_BATCH_FLUSH. > > > > TTU_SPLIT_FREEZE is a special case of migration used when splitting an > > anonymous page. This is most easily dealt with by calling the correct > > function from unmap_page() in mm/huge_memory.c - either > > try_to_migrate() for PageAnon or try_to_unmap(). > > > > Signed-off-by: Alistair Popple <apopple@xxxxxxxxxx> > > Reviewed-by: Christoph Hellwig <hch@xxxxxx> > > Looks reasonable to me. I do worry a bit about code duplication. I was initially concerned about this when splitting try_to_unmap_one() up but most of the code paths ended up being pretty orthogonal and I think the clarity gained from separating them is worth a small amount of duplication. > At some point in the future, it might be discovered that other combinations > of TTU_XXX flags are needed in which case a careful check of try_to_migrate() > and try_to_unmap() will be needed. I wanted to keep the code as simple as possible by removing all dead code paths that that weren't in use today. I think this is likely to be more of an issue if new TTU_XXX flags are added as there are already explicit (and hopefully exhaustive) checks in try_to_migrate() and try_to_unmap() for unsupported combinations of existing flags. To avoid ending up in the same situation again I would rather not have more TTU_XXX flags added if at all possible though. > Reviewed-by: Ralph Campbell <rcampbell@xxxxxxxxxx> Thanks. - Alistair
