On 9/3/24 6:25 PM, John Hubbard wrote:
On 9/3/24 11:19 AM, Suren Baghdasaryan wrote:
On Sun, Sep 1, 2024 at 10:16 PM Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
On Sun, 1 Sep 2024 21:41:28 -0700 Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
...
We shouldn't be offering things like this to our users. If we cannot decide, how
can they?
Thinking about the ease of use, the CONFIG_PGALLOC_TAG_REF_BITS is the
hardest one to set. The user does not know how many page allocations
I should probably clarify my previous reply, so here is the more detailed
version:
are there. I think I can simplify this by trying to use all unused
page flag bits for addressing the tags. Then, after compilation we can
Yes.
follow the rules I mentioned before:
- If the available bits are not enough to address all kernel page
allocations, we issue an error. The user should disable
CONFIG_PGALLOC_TAG_USE_PAGEFLAGS.
The configuration should disable itself, in this case. But if that is
too big of a change for now, I suppose we could fall back to an error
message to the effect of, "please disable CONFIG_PGALLOC_TAG_USE_PAGEFLAGS
because the kernel build system is still too primitive to do that for you". :)
- If there are enough unused bits but we have to push last_cpupid out
of page flags, we issue a warning and continue. The user can disable
CONFIG_PGALLOC_TAG_USE_PAGEFLAGS if last_cpupid has to stay in page
flags.
Let's try to decide now, what that tradeoff should be. Just pick one based
on what some of us perceive to be the expected usefulness and frequency of
use between last_cpuid and these tag refs.
If someone really needs to change the tradeoff for that one bit, then that
someone is also likely able to hack up a change for it.
thanks,
--
John Hubbard
- If we run out of addressing space during module loading, we disable
allocation tagging and continue. The user should disable
CONFIG_PGALLOC_TAG_USE_PAGEFLAGS.
If the computer already knows what to do, it should do it, rather than
prompting the user to disable a deeply mystifying config parameter.
This leaves one outstanding case:
- If we run out of addressing space during module loading but we would
not run out of space if we pushed last_cpupid out of page flags during
compilation.
In this case I would want the user to have an option to request a
larger addressing space for page allocation tags at compile time.
Maybe I can keep CONFIG_PGALLOC_TAG_REF_BITS for such explicit
requests for a larger space? This would limit the use of
CONFIG_PGALLOC_TAG_REF_BITS to this case only. In all other cases the
number of bits would be set automatically. WDYT?
Manually dealing with something like this is just not going to work.
The more I read this story, the clearer it becomes that this should be
entirely done by the build system: set it, or don't set it, automatically.
And if you can make it not even a kconfig item at all, that's probably even
better.
And if there is no way to set it automatically, then that probably means
that the feature is still too raw to unleash upon the world.
thanks,
