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So, when no offload >>>>> driver is active, the branch is never taken and everything goes >>>>> through the standard path. >>>>> >>>>> After TLB flush, batch copy the eligible folios via folios_mc_copy() >>>>> and pass already_copied=true into migrate_folios_move() so >>>>> __migrate_folio() skips the per-folio copy. >>>>> >>>>> On batch copy failure, already_copied flag stays false and each folio >>>>> fall back to individual copy. >>>>> >>>>> Signed-off-by: Shivank Garg >>>>> --- >>>>> include/linux/mm.h | 2 ++ >>>>> mm/migrate.c | 61 +++++++++++++++++++++++++++++++++++----------- >>>>> mm/util.c | 30 +++++++++++++++++++++++ >>>>> 3 files changed, 79 insertions(+), 14 deletions(-) >>>> >>>> [...] >>>> >>>>> >>>>> +DEFINE_STATIC_KEY_FALSE(migrate_offload_enabled); >>>>> + >>>>> static const struct movable_operations *offline_movable_ops; >>>>> static const struct movable_operations *zsmalloc_movable_ops; >>>>> >>>>> @@ -1724,6 +1727,12 @@ static int migrate_hugetlbs(struct list_head *from, new_folio_t get_new_folio, >>>>> return nr_failed; >>>>> } >>>>> >>>>> +/* movable_ops folios have their own migrate path */ >>>>> +static bool folio_supports_batch_copy(struct folio *folio) >>>>> +{ >>>>> + return likely(!page_has_movable_ops(&folio->page)); >>>>> +} >>>> >>>> As these things are not actually folios (and callers will have to be taught to >>>> distinguish them way, way earlier), I guess you should make this >>>> >>>> /* movable_ops pages have a separate migration path */ >>>> static bool page_supports_batch_copy(struct page *page) >>>> ... >>> >>> I'll change this. >>> >>>> >>>>> + >>>>> static void migrate_folios_move(struct list_head *src_folios, >>>>> struct list_head *dst_folios, >>>>> free_folio_t put_new_folio, unsigned long private, >>>>> @@ -1752,7 +1761,7 @@ static void migrate_folios_move(struct list_head *src_folios, >>>>> /* >>>>> * The rules are: >>>>> * 0: folio will be freed >>>>> - * -EAGAIN: stay on the unmap_folios list >>>>> + * -EAGAIN: stay on the src_folios list >>>>> * Other errno: put on ret_folios list >>>>> */ >>>>> switch (rc) { >>>> >>>> >>>> [...] >>>> >>>>> --- a/mm/util.c >>>>> +++ b/mm/util.c >>>>> @@ -778,6 +778,36 @@ int folio_mc_copy(struct folio *dst, struct folio *src) >>>>> } >>>>> EXPORT_SYMBOL(folio_mc_copy); >>>>> >>>>> +/** >>>>> + * folios_mc_copy - Copy the contents of list of folios. >>>>> + * @dst_list: destination folio list. >>>>> + * @src_list: source folio list. >>>>> + * @folios_cnt: unused here, present for callback signature compatibility. >>>>> + * >>>>> + * Walks list of src and dst folios in lockstep and copies folio >>>>> + * content via folio_mc_copy(). The caller must ensure both lists have >>>>> + * the same number of entries. This may sleep. >>>> >>>> This *function* >>>> >>> >>> Will add function. >>> >>>>> + * >>>>> + * Return: 0 on success, negative errno on failure. >>>>> + */ >>>>> +int folios_mc_copy(struct list_head *dst_list, struct list_head *src_list, >>>>> + unsigned int __always_unused folios_cnt) >>>>> +{ >>>>> + struct folio *src, *dst; >>>>> + int ret; >>>>> + >>>>> + dst = list_first_entry(dst_list, struct folio, lru); >>>>> + list_for_each_entry(src, src_list, lru) { >>>>> + ret = folio_mc_copy(dst, src); >>>>> + if (ret) >>>>> + return ret; >>>>> + dst = list_next_entry(dst, lru); >>>>> + } >>>> >>>> Wouldn't it be cleaner to remember "already copied" immediately after we ... >>>> performed the copy? (succeeded with folio_mc_copy) >>> >>> IIUC, you mean something like: >>> list_for_each_entry(src, src_list, lru) { >>> ret = folio_mc_copy(dst, src); >>> if (ret) >>> return ret; >>> dst->migrate_info |= FOLIO_CONTENT_COPIED; >>> dst = list_next_entry(dst, lru); >>> } >>> >>> This will be cleaner. However, it will make the folios_mc_copy() specific for >>> page migration and can't be reused by other potential callers. >>> And offload driver (like DCBM) must set the already copied marker >>> themselves for successful copies. >> >> Right, maybe some helper for that would be possible? >> I'm adding a migrate.c specific implementation: migrate_folios_mc_copy(). >>> This will have an upside that on partial-success (even a single failed copy), >>> it will no longer forces re-copy of the whole batch. >>> If that's the intention, I'll move this to migrate.c itself. >> >> True. I'd probably have to take a look at the end result to see if it looks >> better that way. > > Maybe do this optimization in a separate patch to make it simpler to review. > > BTW, the whole migration code has become more and more complicated due to > batching at different levels, I wonder if it is worth the complication to > optimize for the rare memory error case. I agree on complication, but I think this change is actually simpler than my previous patch. Using per-folio marker lets me drop the already_copied arg that was passed through different function: migrate_folios_move() -> migrate_folio_move() -> move_to_new_folio(). And there is no separate error path with this. Whoever performs the batch copy (migrate_folios_mc_copy() or a driver callback) sets FOLIO_CONTENT_COPIED on each dst it successfully copied, and __migrate_folio() reads that marker to skip folio_mc_copy(). If copy is unsuccessful (e.g. memory error), FOLIO_CONTENT_COPIED is NOT marked, and __migrate_folio() will not skip the folio_mc_copy(). Since the per-folio marking is the natural implementation for this, I kept it in this patch (af44801ee). Git Tree for reference: https://github.com/AMDESE/linux-mm/commits/shivank/batch-migrate-offload-v6-wip Thanks, Shivank