On 2 Mar 2024, at 20:17, Yu Zhao wrote: > On Thu, Feb 29, 2024 at 4:55 PM Zi Yan <ziy@xxxxxxxxxx> wrote: >> >> On 29 Feb 2024, at 13:34, Yu Zhao wrote: >> >>> In contrast to split, shatter migrates occupied pages in a partially >>> mapped THP to a bunch of base folios. IOW, unlike split done in place, >>> shatter is the exact opposite of collapse. >>> >>> The advantage of shattering is that it keeps the original THP intact. >> >> Why keep the THP intact? To prevent the THP from fragmentation, since >> the shattered part will not be returned to buddy allocator for reuse? > > There might be a confusion here: there is no "shattered part" -- the > entire THP becomes free after shattering (the occupied part is moved > to a bunch of 4KB pages). Got it. Now it makes more sense. Thanks. > >> I agree with the idea of shattering, but keeping THP intact might >> give us trouble for 1GB THP case when PMD mapping is created after >> shattering. How to update mapcount for a PMD mapping in the middle of >> a 1GB folio? I used head[0], head[512], ... as the PMD mapping head >> page, but that is ugly. For mTHPs, there is no such problem since >> only PTE mappings are involved. > > If we don't consider the copying cost during shattering, it can work > for 1GB THPs as it does for 2MB THPs. > >> It might be better to just split the THP and move free pages to a >> donot-use free list until the rest are freed too > > The main reason we do shattering is, using a crude analogy, a million > dollar in $10,000 bills (yes, they exist) is worth a lot more than > that in pennies. You can carry the former in your pocket but the > latter weighs at least 250 tons. So if we split, we lose money. > > 1GB THP is one of the important *end goals* for TAO. But I don't want > to go into details since we need to focus on the first few steps at > the current stage. > > The problem with shattering for 1GB is the copying cost -- if we > shatter a 1GB THP half mapped/unmapped, we'd have to copy 512MB data, > which is unacceptable. 1GB THP requires something we call "THP > fungibility" (see the epilogue) -- we do split in place, but we also > "collapse" in place (called THP recovery, i.e., MADV_RECOVERY). > > Shattering is for 2MB THPs only. Got it. Thanks for the explanation. > > >> if the zone enforces >> a minimal order that is larger than the free pages. >> >>> The cost of copying during the migration is not a side effect, but >>> rather by design, since splitting is considered a discouraged >>> behavior. In retail terms, the return of a purchase is charged with a >>> restocking fee and the original goods can be resold. >>> >>> THPs from ZONE_NOMERGE can only be shattered, since they cannot be >>> split or merged. THPs from ZONE_NOSPLIT can be shattered or split (the >>> latter requires [1]), if they are above the minimum order. >>> >>> [1] https://lore.kernel.org/20240226205534.1603748-1-zi.yan@xxxxxxxx/ >>> >> >> >> -- >> Best Regards, >> Yan, Zi -- Best Regards, Yan, Zi
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