RE: [PATCH v3 4/7] swiotlb: if swiotlb is full, fall back to a transient memory pool

["Michael Kelley (LINUX)" <mikelley@microsoft.com>]

From: Greg Kroah-Hartman <gregkh@linuxfoundation.org> 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@huaweicloud.com> 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.

Michael