On 2022-03-04 09:41, Joerg Roedel wrote:
On Fri, Mar 04, 2022 at 07:36:46AM +0800, Miles Chen wrote:
Hi Robin,
For various reasons based on the allocator behaviour and typical
use-cases at the time, when the max32_alloc_size optimisation was
introduced it seemed reasonable to couple the reset of the tracked
size to the update of cached32_node upon freeing a relevant IOVA.
However, since subsequent optimisations focused on helping genuine
32-bit devices make best use of even more limited address spaces, it
is now a lot more likely for cached32_node to be anywhere in a "full"
32-bit address space, and as such more likely for space to become
available from IOVAs below that node being freed.
At this point, the short-cut in __cached_rbnode_delete_update() really
doesn't hold up any more, and we need to fix the logic to reliably
provide the expected behaviour. We still want cached32_node to only move
upwards, but we should reset the allocation size if *any* 32-bit space
has become available.
Reported-by: Yunfei Wang <yf.wang@xxxxxxxxxxxx>
Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
Would you mind adding:
Cc: <stable@xxxxxxxxxxxxxxx>
Applied without stable tag for now. If needed, please consider
re-sending it for stable when this patch is merged upstream.
Yeah, having figured out the history, I ended up with the opinion that
it was a missed corner-case optimisation opportunity, rather than an
actual error with respect to intent or implementation, so I
intentionally left that out. Plus figuring out an exact Fixes tag might
be tricky - as above I reckon it probably only started to become
significant somwehere around 5.11 or so.
All of these various levels of retry mechanisms are only a best-effort
thing, and ultimately if you're making large allocations from a small
space there are always going to be *some* circumstances that still
manage to defeat them. Over time, we've made them try harder, but that
fact that we haven't yet made them try hard enough to work well for a
particular use-case does not constitute a bug. However as Joerg says,
anyone's welcome to make a case to Greg to backport a mainline commit if
it's a low-risk change with significant benefit to real-world stable
kernel users.
Thanks all!
Robin.
