On 02/01/25 5:13 pm, Ryan Roberts wrote:
On 30/12/2024 16:36, Zi Yan wrote:
On Mon Dec 30, 2024 at 2:09 AM EST, Dev Jain wrote:
On 20/12/24 4:35 pm, Ryan Roberts wrote:
On 18/12/2024 09:50, Dev Jain wrote:
On 18/12/24 2:33 pm, Ryan Roberts wrote:
On 16/12/2024 16:51, Dev Jain wrote:
One of the testcases triggers a CoW on the 255th page (0-indexing) with
max_ptes_shared = 256. This leads to 0-254 pages (255 in number) being unshared,
and 257 pages shared, exceeding the constraint. Suppose we run the test as
./khugepaged -s 2. Therefore, khugepaged starts collapsing the range to order-2
folios, since PMD-collapse will fail due to the constraint.
When the scan reaches 254-257 PTE range, because at least one PTE in this range
is writable, with other 3 being read-only, khugepaged collapses this into an
order-2 mTHP, resulting in 3 extra PTEs getting unshared. After this, we
encounter
a 4-sized chunk of read-only PTEs, and mTHP collapse stops according to the
scaled
constraint, but the number of shared PTEs have now come under the constraint for
PMD-sized THPs. Therefore, the next scan of khugepaged will be able to collapse
this range into a PMD-mapped hugepage, leading to failure of this subtest. Fix
this by reducing the CoW range.
Is this description essentially saying that it's now possible to creep towards
collapsing to a full PMD-size block over successive scans due to rounding errors
in the scaling? Or is this just trying an edge case and the problem doesn't
generalize?
For this case, max_ptes_shared for order-2 is 256 >> (9 - 2) = 2, without
rounding errors. We cannot
really get a rounding problem because we are rounding down, essentially either
keep the restriction
same, or making it stricter, as we go down the orders.
But thinking again, this behaviour may generalize: essentially, let us say that
the distribution
of none ptes vs filled ptes is very skewed for the PMD case. In a local region,
this distribution
may not be skewed, and then an mTHP collapse will occur, making this entire
region uniform. Over time this
may keep happening and then the region will become globally uniform to come
under the PMD-constraint
on max_ptes_none, and eventually PMD-collapse will occur. Which may beg the
question whether we want
to detach khugepaged orders from mTHP sysfs settings.
We want to avoid new user controls at all costs, I think.
I think an example of the problem you are describing is: Let's say we start off
with all mTHP orders enabled and max_ptes_none is 50% (so 256). We have a 2M VMA
aligned over a single PMD. The first 2 4K pages of the VMA are allocated.
khugepaged will scan this VMA and decide to collapse the first 4 PTEs to a
single order-2 (16K) folio; that's allowed because 50% of the PTEs were none.
But now on the next scan, 50% of the first 8 PTEs are none so it will collapse
to 32K. Then on the next scan it will collapse to 64K, and so on all the way to
2M. So by faulting in 2 pages originally we have now collapsed to 2M dispite the
control trying to prevent it, and we have done it in a very inefficient way.
If max_ptes_none was 75% you would only need every other order enabled (I think?).
In practice perhaps it's not a problem because you are only likely to have 1 or
2 mTHP sizes enabled. But I wonder if we need to think about how to protect from
this "creep"?
Perhaps only consider a large folio for collapse into a larger folio if it
wasn't originally collapsed by khugepaged in the first place? That would need a
folio flag... and I suspect that will cause other edge case issues if we think
about it for 5 mins...
Another way of thinking about it is; if all the same mTHP orders were enabled at
fault time (and the allocation suceeded) we would have allocated the largest
order anyway, so the end states are the same. But the large number of
incremental collapses that khugepaged will perform feel like a problem.
I'm not quite sure what the answer is.
Can't really think of anything else apart from decoupling khugepaged sysfs from mTHP sysfs...
One (not so effective) workaround is to add a VMA flag to make
khugepaged to skip scanning a VMA that khugepaged has collapsed before
and reset the flag in a future page fault. This would prevent khugepaged
from doing this "creep" collapse behavior until the page tables covered
by the VMA is changed. This is not perfect, since the page faults might
not change the aforementioned region and later khugepaged can still
perform the "creep".
I guess what you really want is a bitmap with a bit per page in the VMA to tell
you whether it exists due to fault or collapse. But... yuk...
It looks like Ubuntu at least is not modifying the default max_ptes_none, which
is 511. So for general purpose distros I guess we won't see this issue in
practice because we will always collapse to the largest enabled size.
Will still see this problem for max_ptes_shared = 256...
We can set this to 255, so that the fraction progress as follows:
255/512 -> 127/256 -> 63/128 -> 31/64 -> 15/32 -> 7/16 -> 3/8 -> 1/4 -> 0/2
This is the best possible fractional decrease we can get since we always end
up on an odd number and lose a 1 due to rounding.
The only real solution seems to be to track whether the PTE/page we have is original
or collapsed.
So with that in mind, perhaps Zi's suggested single VM flag idea will be good
enough?
Thanks,
Ryan
