Hugh Dickins <hughd@xxxxxxxxxx> writes: > On Tue, 23 May 2023, Alistair Popple wrote: >> Hugh Dickins <hughd@xxxxxxxxxx> writes: >> >> > migrate_vma_collect_pmd(): remove the pmd_trans_unstable() handling after >> > splitting huge zero pmd, and the pmd_none() handling after successfully >> > splitting huge page: those are now managed inside pte_offset_map_lock(), >> > and by "goto again" when it fails. >> > >> > But the skip after unsuccessful split_huge_page() must stay: it avoids an >> > endless loop. The skip when pmd_bad()? Remove that: it will be treated >> > as a hole rather than a skip once cleared by pte_offset_map_lock(), but >> > with different timing that would be so anyway; and it's arguably best to >> > leave the pmd_bad() handling centralized there. >> >> So for a pmd_bad() the sequence would be: >> >> 1. pte_offset_map_lock() would return NULL and clear the PMD. >> 2. goto again marks the page as a migrating hole, >> 3. In migrate_vma_insert_page() a new PMD is created by pmd_alloc(). >> 4. This leads to a new zero page getting mapped for the previously >> pmd_bad() mapping. > > Agreed. > >> >> I'm not entirely sure what the pmd_bad() case is used for but is that >> ok? I understand that previously it was all a matter of timing, but I >> wouldn't rely on the previous code being correct in this regard either. > > The pmd_bad() case is for when the pmd table got corrupted (overwritten, > cosmic rays, whatever), and that pmd entry is easily recognized as > nonsense: we try not to crash on it, but user data may have got lost. > > My "timing" remark may not be accurate: I seem to be living in the past, > when we had a lot more "pmd_none_or_clear_bad()"s around than today - I > was thinking that any one of them could be racily changing the bad to none. > Though I suppose I am now making my timing remark accurate, by changing > the bad to none more often again. > > Since data is liable to be lost anyway (unless the corrupted entry was > actually none before it got corrupted), it doesn't matter greatly what > we do with it (some would definitely prefer a crash, but traditionally > we don't): issue a "pmd bad" message and not get stuck in a loop is > the main thing. Thanks for the background. Either skipping it or marking it as a hole as you've done here will avoid a loop so feel free to add: Reviewed-by: Alistair Popple <apopple@xxxxxxxxxx> >> >> > migrate_vma_insert_page(): remove comment on the old pte_offset_map() >> > and old locking limitations; remove the pmd_trans_unstable() check and >> > just proceed to pte_offset_map_lock(), aborting when it fails (page has >> > now been charged to memcg, but that's so in other cases, and presumably >> > uncharged later). >> >> Correct, the non-migrating page will be freed later via put_page() which >> will uncharge the page. > > Thanks for confirming, yes, it was more difficult once upon a time, > but nowadays just a matter of reaching the final put_page() > > Hugh
