On 13.11.24 03:16, Yafang Shao wrote:
On Tue, Nov 12, 2024 at 11:19 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
Sorry, but this code is getting quite confusing, especially with such
misleading "large folio" comments.
Even without MADV_HUGEPAGE we will be allocating large folios, as
emphasized by Willy [1]. So the only thing MADV_HUGEPAGE controls is
*which* large folios we allocate. .. as Willy says [2]: "We were only
intending to breach the 'max' for the MADV_HUGE case, not for all cases."
I have no idea how *anybody* should derive from the code here that we
treat MADV_HUGEPAGE in a special way.
Simply completely confusing.
My interpretation of "I don't know if we should try to defend a stupid
sysadmin against the consequences of their misconfiguration like this"
means" would be "drop this patch and don't change anything".
Without this change, large folios won’t function as expected.
Currently, to support MADV_HUGEPAGE, you’d need to set readahead_kb to
2MB, 4MB, or more. However, many applications run without
> MADV_HUGEPAGE, and a larger readahead_kb might not be optimal for> them.
Someone configured: "Don't readahead more than 128KiB"
And then we complain why we "don't readahead more than 128KiB".
That is just bikeshielding.
It's called "reading the documentation and trying to make sense of a
patch". ;)
So, what’s your suggestion? Simply setting readahead_kb to 2MB? That
would almost certainly cause issues elsewhere.
I'm not 100% sure. I'm trying to make sense of it all.
And I assume there is a relevant difference now between "readahead 2M
using all 4k pages" and "readahead 2M using a single large folio".
I agree that likely readahead using many 4k pages is a worse idea than
just using a single large folio ... if we manage to allocate one. And
it's all not that clear in the code ...
FWIW, I looked at "read_ahead_kb" values on my Fedora40 notebook and
they are all set to 128KiB. I'm not so sure if they really should be
that small ... or if large folio readahead code should just be able to
exceed it.
"mm/filemap: Support VM_HUGEPAGE for file mappings" talks about "even if
we have no history of readahead being successful".
So not about exceeding the configured limit, but exceeding the
"readahead history".
So I consider VM_HUGEPAGE the sign here to "ignore readahead history"
and not to "violate the config".
MADV_HUGEPAGE is definitely a new addition to readahead, and its
behavior isn’t yet defined in the documentation. All we need to do is
clarify its behavior there. The documentation isn’t set in stone—we
can update it as long as it doesn’t disrupt existing applications.
If Willy thinks this is the way to go, then we should document that
MADV_HUGEPAGE may ignore the parameter, agreed.
I still don't understand your one comment:
"It's worth noting that if read_ahead_kb is set to a larger value that
isn't aligned with huge page sizes (e.g., 4MB + 128KB), it may still
fail to map to hugepages."
Do you mean that MADV_HUGEPAGE+read_ahead_kb<=4M will give you 2M pages,
but MADV_HUGEPAGE+read_ahead_kb>4M won't? Or is this the case without
MADV_HUGEPAGE?
If MADV_HUGEPAGE ignores read_ahead_kb completely, it's easy to document.
But that's just my opinion.
No changes to API, no confusing code.
New features like large folios can often create confusion with
existing rules or APIs, correct?
We should not try making it even more confusing, if possible.
A quick tip for you: the readahead size already exceeds readahead_kb
even without MADV_HUGEPAGE. You might want to spend some time tracing
that behavior.
Care to save me some time and point me at what you mean?
In summary, it’s really the readahead code itself that’s causing the
confusion—not MADV_HUGEPAGE.
Let me dig into the code in more detail if can make sense of it all.
--
Cheers,
David / dhildenb
