On Fri, 7 Jul 2023 10:29:00 +0100 Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > On Thu, Jul 06, 2023 at 02:22:50PM +0000, Michael Kelley (LINUX) wrote: > > From: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Sent: Thursday, July 6, 2023 1:07 AM > > > > > > On Thu, Jul 06, 2023 at 03:50:55AM +0000, Michael Kelley (LINUX) wrote: > > > > From: Petr Tesarik <petrtesarik@xxxxxxxxxxxxxxx> Sent: Tuesday, June 27, 2023 > > > 2:54 AM > > > > > > > > > > Try to allocate a transient memory pool if no suitable slots can be found, > > > > > except when allocating from a restricted pool. The transient pool is just > > > > > enough big for this one bounce buffer. It is inserted into a per-device > > > > > list of transient memory pools, and it is freed again when the bounce > > > > > buffer is unmapped. > > > > > > > > > > Transient memory pools are kept in an RCU list. A memory barrier is > > > > > required after adding a new entry, because any address within a transient > > > > > buffer must be immediately recognized as belonging to the SWIOTLB, even if > > > > > it is passed to another CPU. > > > > > > > > > > Deletion does not require any synchronization beyond RCU ordering > > > > > guarantees. After a buffer is unmapped, its physical addresses may no > > > > > longer be passed to the DMA API, so the memory range of the corresponding > > > > > stale entry in the RCU list never matches. If the memory range gets > > > > > allocated again, then it happens only after a RCU quiescent state. > > > > > > > > > > Since bounce buffers can now be allocated from different pools, add a > > > > > parameter to swiotlb_alloc_pool() to let the caller know which memory pool > > > > > is used. Add swiotlb_find_pool() to find the memory pool corresponding to > > > > > an address. This function is now also used by is_swiotlb_buffer(), because > > > > > a simple boundary check is no longer sufficient. > > > > > > > > > > The logic in swiotlb_alloc_tlb() is taken from __dma_direct_alloc_pages(), > > > > > simplified and enhanced to use coherent memory pools if needed. > > > > > > > > > > Note that this is not the most efficient way to provide a bounce buffer, > > > > > but when a DMA buffer can't be mapped, something may (and will) actually > > > > > break. At that point it is better to make an allocation, even if it may be > > > > > an expensive operation. > > > > > > > > I continue to think about swiotlb memory management from the standpoint > > > > of CoCo VMs that may be quite large with high network and storage loads. > > > > These VMs are often running mission-critical workloads that can't tolerate > > > > a bounce buffer allocation failure. To prevent such failures, the swiotlb > > > > memory size must be overly large, which wastes memory. > > > > > > If "mission critical workloads" are in a vm that allowes overcommit and > > > no control over other vms in that same system, then you have worse > > > problems, sorry. > > > > > > Just don't do that. > > > > > > > No, the cases I'm concerned about don't involve memory overcommit. > > > > CoCo VMs must use swiotlb bounce buffers to do DMA I/O. Current swiotlb > > code in the Linux guest allocates a configurable, but fixed, amount of guest > > memory at boot time for this purpose. But it's hard to know how much > > swiotlb bounce buffer memory will be needed to handle peak I/O loads. > > This patch set does dynamic allocation of swiotlb bounce buffer memory, > > which can help avoid needing to configure an overly large fixed size at boot. > > But, as you point out, memory allocation can fail at runtime, so how can > you "guarantee" that this will work properly anymore if you are going to > make it dynamic? In general, there is no guarantee, of course, because bounce buffers may be requested from interrupt context. I believe Michael is looking for the SWIOTLB_MAY_SLEEP flag that was introduced in my v2 series, so new pools can be allocated with GFP_KERNEL instead of GFP_NOWAIT if possible, and then there is no need to dip into the coherent pool. Well, I have deliberately removed all complexities from my v3 series, but I have more WIP local topic branches in my local repo: - allow blocking allocations if possible - allocate a new pool before existing pools are full - free unused memory pools I can make a bigger series, or I can send another series as RFC if this is desired. ATM I don't feel confident enough that my v3 series will be accepted without major changes, so I haven't invested time into finalizing the other topic branches. @Michael: If you know that my plan is to introduce blocking allocations with a follow-up patch series, is the present approach acceptable? Petr T
