On 06/03/25 00:47, Ritesh Harjani (IBM) wrote:
Sourabh Jain <sourabhj...@linux.ibm.com> writes:
Hello Ritesh,
On 04/03/25 10:27, Ritesh Harjani (IBM) wrote:
Sourabh Jain <sourabhj...@linux.ibm.com> writes:
Hello Ritesh,
Thanks for the review.
On 02/03/25 12:05, Ritesh Harjani (IBM) wrote:
Sourabh Jain <sourabhj...@linux.ibm.com> writes:
The fadump kernel boots with limited memory solely to collect the kernel
core dump. Having gigantic hugepages in the fadump kernel is of no use.
Sure got it.
Many times, the fadump kernel encounters OOM (Out of Memory) issues if
gigantic hugepages are allocated.
To address this, disable gigantic hugepages if fadump is active by
returning early from arch_hugetlb_valid_size() using
hugepages_supported(). When fadump is active, the global variable
hugetlb_disabled is set to true, which is later used by the
PowerPC-specific hugepages_supported() function to determine hugepage
support.
Returning early from arch_hugetlb_vali_size() not only disables
gigantic hugepages but also avoids unnecessary hstate initialization for
every hugepage size supported by the platform.
kernel logs related to hugepages with this patch included:
kernel argument passed: hugepagesz=1G hugepages=1
First kernel: gigantic hugepage got allocated
==============================================
dmesg | grep -i "hugetlb"
-------------------------
HugeTLB: registered 1.00 GiB page size, pre-allocated 1 pages
HugeTLB: 0 KiB vmemmap can be freed for a 1.00 GiB page
HugeTLB: registered 2.00 MiB page size, pre-allocated 0 pages
HugeTLB: 0 KiB vmemmap can be freed for a 2.00 MiB page
$ cat /proc/meminfo | grep -i "hugetlb"
-------------------------------------
Hugetlb: 1048576 kB
Was this tested with patch [1] in your local tree?
[1]:
https://web.git.kernel.org/pub/scm/linux/kernel/git/powerpc/linux.git/commit/?id=d629d7a8efc33
IIUC, this patch [1] disables the boot time allocation of hugepages.
Isn't it also disabling the boot time allocation for gigantic huge pages
passed by the cmdline params like hugepagesz=1G and hugepages=2 ?
Yes, I had the patch [1] in my tree.
My understanding is that gigantic pages are allocated before normal huge
pages.
In hugepages_setup() in hugetlb.c, we have:
if (hugetlb_max_hstate && hstate_is_gigantic(parsed_hstate))
hugetlb_hstate_alloc_pages(parsed_hstate);
I believe the above code allocates memory for gigantic pages, and
hugetlb_init() is
called later because it is a subsys_initcall.
So, by the time the kernel reaches hugetlb_init(), the gigantic pages
are already
allocated. Isn't that right?
Please let me know your opinion.
Yes, you are right. We are allocating hugepages from memblock, however
this isn't getting advertized anywhere. i.e. there is no way one can
know from any user interface on whether hugepages were allocated or not.
i.e. for fadump kernel when hugepagesz= and hugepages= params are
passed, though it will allocate gigantic pages, it won't advertize this
in meminfo or anywhere else. This was adding the confusion when I tested
this (which wasn't clear from the commit msg either).
And I guess this is happening during fadump kernel because of our patch
[1], which added a check to see whether hugetlb_disabled is true in
hugepages_supported(). Due to this hugetlb_init() is now not doing the
rest of the initialization for those gigantic pages which were allocated
due to cmdline options from hugepages_setup().
[1]:
https://lore.kernel.org/linuxppc-dev/20241202054310.928610-1-sourabhj...@linux.ibm.com/
Now as we know from below that fadump can set hugetlb_disabled call in
early_setup().
i.e. fadump can mark hugetlb_disabled to true in
early_setup() -> early_init_devtree() -> fadump_reserve_mem()
And hugepages_setup() and hugepagesz_setup() gets called late in
start_kernel() -> parse_args()
And we already check for hugepages_supported() in all necessary calls in
mm/hugetlb.c. So IMO, this check should go in mm/hugetlb.c in
hugepagesz_setup() and hugepages_setup(). Because otherwise every arch
implementation will end up duplicating this by adding
hugepages_supported() check in their arch implementation of
arch_hugetlb_valid_size().
e.g. references of hugepages_supported() checks in mm/hugetlb.c
mm/hugetlb.c hugetlb_show_meminfo_node 4959 if (!hugepages_supported())
mm/hugetlb.c hugetlb_report_node_meminfo 4943 if (!hugepages_supported())
mm/hugetlb.c hugetlb_report_meminfo 4914 if (!hugepages_supported())
mm/hugetlb.c hugetlb_overcommit_handler 4848 if (!hugepages_supported())
mm/hugetlb.c hugetlb_sysctl_handler_common 4809 if (!hugepages_supported())
mm/hugetlb.c hugetlb_init 4461 if (!hugepages_supported()) {
mm/hugetlb.c dissolve_free_hugetlb_folios 2211 if (!hugepages_supported())
fs/hugetlbfs/inode.c init_hugetlbfs_fs 1604 if (!hugepages_supported()) {
Let me also see the history on why this wasn't done earlier though...
... Oh actually there is more history to this. See [2]. We already had
hugepages_supported() check in hugepages_setup() and other places
earlier which was removed to fix some other problem in ppc ;)
[2]:
https://web.git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c2833a5bf75b3657c4dd20b3709c8c702754cb1f
Hence I believe this needs a wider cleanup than just fixing it for our
arch. I see there is a patch series already fixing these code paths,
which is also cleaning up the path of gigantic hugepage allocation in
hugepages_setup(). I think it is in mm-unstable branch too. Can we
please review & test that to make sure that the fadump usecase of
disabling hugepages & gigantic are getting covered properly?
[3]: https://lore.kernel.org/all/20250228182928.2645936-1-f...@google.com/
I evaluated the patch series [3] for the fadump issue, and here are my
observations:
Currently, the patch series [3] does not fix the issue I am trying to
address with this patch.
With patch series [3] applied, I see the following logs when booting the
fadump kernel with
hugepagesz=1G hugepages=1
|
|
With just Patch series [3]:
------------------------------------
kdump:/# dmesg | grep -i HugeTLB
[ 0.000000] HugeTLB: allocating 10 of page size 1.00 GiB failed.
Only allocated 9 hugepages.
[ 0.405964] HugeTLB support is disabled!
[ 0.409162] HugeTLB: huge pages not supported, ignoring associated
command-line parameters
[ 0.437740] hugetlbfs: disabling because there are no supported
hugepage sizes
One good thing is that the kernel now at least reports the gigantic
pages allocated, which was
not the case before. I think patch series [3] introduced that improvement.
Now, on top of patch series [3], I applied this fix, and the kernel
prints the following logs:
Patch series [3] + this fix:
------------------------------------
[ 0.000000] HugeTLB: unsupported hugepagesz=1G
[ 0.000000] HugeTLB: hugepages=10 does not follow a valid hugepagesz,
ignoring
[ 0.366158] HugeTLB support is disabled!
[ 0.398004] hugetlbfs: disabling because there are no supported
hugepage sizes
With these logs, one can clearly identify what is happening.
What are your thoughts on this fix now?
Do you still think handling this in generic code is better?
I believe so yes (unless we have a valid reason for not doing that).
hugepages_supported() is an arch specific call. If you see the prints
above what we are essentially saying is that this is not a valid
hugepagesz. But that's not the case really right. What it should just
reflect is that the hugepages support is disabled.
Yeah better to just print hugetlb support is disabled.
That being said, I will have to go and look into that series to suggest,
where in that path it should use hugepages_supported() arch call to see
if the hugepages are supported or not before initializing. And hopefully
as you suggested since our cmdline parsing problem was already solved,
it should not be a problem in using hugepages_supported() during cmdline
parsing phase.
But let me check that series and get back.
Given that I was already advised to handle things in arch
code. [4]
Or should we keep it this way?
One advantage handling things in arch_hugetlb_valid_size() is that it helps
avoid populating hstates since it is not required anyway. I am not claiming
that it is not possible in generic code.
IMO, even adding hugepages_supported() check at the right place should avoid
populating hstates too. But let's confirm that.
Agree I will explore that.
Thanks,
Sourabh Jain
-ritesh
Thoughts?
Thanks,
Sourabh Jain
[3]: https://lore.kernel.org/all/20250228182928.2645936-1-f...@google.com/
[4]: https://lore.kernel.org/all/20250122150613.28a92438@thinkpad-T15/
