Re: [PATCH 01/16] PCI/P2PDMA: Pass gfp_mask flags to upstream_bridge_distance_warn()

From: Logan Gunthorpe
Date: Mon May 03 2021 - 11:58:03 EST

On 2021-05-01 9:58 p.m., John Hubbard wrote:
> Another odd thing: this used to check for memory failure and just give
> up, and now it doesn't. Yes, I realize that it all still works at the
> moment, but this is quirky and we shouldn't stop here.
> Instead, a cleaner approach would be to push the memory allocation
> slightly higher up the call stack, out to the
> pci_p2pdma_distance_many(). So pci_p2pdma_distance_many() should make
> the kmalloc() call, and fail out if it can't get a page for the seq_buf
> buffer. Then you don't have to do all this odd stuff.

I don't really agree with this assessment. If kmalloc fails to
initialize the seq_buf() (which should be very rare), the only thing
that is lost is the one warning print that tells the user the command
line parameter needed disable the ACS. Everything else works fine,
nothing else can fail. I don't see the need to add extra complexity just
so the code errors out in no-mem instead of just skipping the one,
slightly more informative, warning line.

Also, keep in mind the result of all these functions are cached so it
only ever happens once. So for this to matter, the user would have to do
their first transaction between two devices exactly at the time memory
allocations would fail.

> Furthermore, the call sites can then decide for themselves which GFP
> flags, GFP_ATOMIC or GFP_KERNEL or whatever they want for kmalloc().
> A related thing: this whole exercise would go better if there were a
> preparatory patch or two that changed the return codes in this file to
> something less crazy. There are too many functions that can fail, but
> are treated as if they sort-of-mostly-would-never-fail, in the hopes of
> using the return value directly for counting and such. This is badly
> mistaken, and it leads developers to try to avoid returning -ENOMEM
> (which is what we need here).

Hmm? Which functions can fail? and how?