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).
Yeah I should have added that in my commit message.
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().
Yeah patch [1] has disabled the hugetlb initialization.
[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()) {
v1 proposed to do it in generic code:
https://lore.kernel.org/all/20250121150419.1342794-1-sourabhj...@linux.ibm.com/
But it was suggested to try it in arch specific code.
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
IIUC, I think it was done because the HPAGE_SHIFT was not initialized
early enough
on powrepc. But it is no longer the case after the below commit:
2354ad252b66 ("powerpc/mm: Update default hugetlb size early")
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/
Thoughts?
Agree, if there is cleanup happening already we should try to handle things
there itself. I will review the patch series [3].
Thanks, Ritesh, for the detailed review! Appreciate your time and effort.
Thanks,
Sourabh Jain
-ritesh
Thanks,
Sourabh Jain
HugeTLB: registered 1.00 GiB page size, pre-allocated 1 pages
This print comes from report_hugepages(). The only place from where
report_hugepages() gets called is hugetlb_init(). hugetlb_init() is what
is responsible for hugepages & gigantic hugepage allocations of the
passed kernel cmdline params.
But hugetlb_init() already checks for hugepages_supported() in the very
beginning. So I am not sure whether we need this extra patch to disable
gigantic hugepages allocation by the kernel cmdline params like
hugepagesz=1G and hugepages=2 type of options.
Hence I was wondering if you had this patch [1] in your tree when you were
testing this?
But I may be missing something. Could you please help clarify on whether
we really need this patch to disable gigantic hugetlb page allocations?
Fadump kernel: gigantic hugepage not allocated
===============================================
dmesg | grep -i "hugetlb"
-------------------------
[ 0.000000] HugeTLB: unsupported hugepagesz=1G
[ 0.000000] HugeTLB: hugepages=1 does not follow a valid hugepagesz, ignoring
[ 0.706375] HugeTLB support is disabled!
[ 0.773530] hugetlbfs: disabling because there are no supported hugepage
sizes
$ cat /proc/meminfo | grep -i "hugetlb"
----------------------------------
<Nothing>
What I meant was, when we pass hugepagesz= and hugepages= and fadump=on,
then during the fadump kernel, we still will see the above prints and
nothing in meminfo even w/o this patch (because the user interfaces will
be disabled since hugetlb_supported() is false).
This observation should have been mentioned in the commit msg to avoid
the confusion :)
Cc: Hari Bathini <hbath...@linux.ibm.com>
Cc: Madhavan Srinivasan <ma...@linux.ibm.com>
Cc: Mahesh Salgaonkar <mah...@linux.ibm.com>
Cc: Michael Ellerman <m...@ellerman.id.au>
Cc: Ritesh Harjani (IBM)" <ritesh.l...@gmail.com>
I guess the extra " in the above was not adding me in the cc list.
Hence I missed to see this patch early.
Thanks for point it out. I will fix it.
-ritesh
Reviewed-by: Christophe Leroy <christophe.le...@csgroup.eu>
Signed-off-by: Sourabh Jain <sourabhj...@linux.ibm.com>
---
Changelog:
v1:
https://lore.kernel.org/all/20250121150419.1342794-1-sourabhj...@linux.ibm.com/
v2:
https://lore.kernel.org/all/20250124103220.111303-1-sourabhj...@linux.ibm.com/
- disable gigantic hugepage in arch code, arch_hugetlb_valid_size()
v3:
https://lore.kernel.org/all/20250125104928.88881-1-sourabhj...@linux.ibm.com/
- Do not modify the initialization of the shift variable
v4:
- Update commit message to include how hugepages_supported() detects
hugepages support when fadump is active
- Add Reviewed-by tag
- NO functional change
---
arch/powerpc/mm/hugetlbpage.c | 3 +++
1 file changed, 3 insertions(+)
diff --git a/arch/powerpc/mm/hugetlbpage.c b/arch/powerpc/mm/hugetlbpage.c
index 6b043180220a..88cfd182db4e 100644
--- a/arch/powerpc/mm/hugetlbpage.c
+++ b/arch/powerpc/mm/hugetlbpage.c
@@ -138,6 +138,9 @@ bool __init arch_hugetlb_valid_size(unsigned long size)
int shift = __ffs(size);
int mmu_psize;
+ if (!hugepages_supported())
+ return false;
+
/* Check that it is a page size supported by the hardware and
* that it fits within pagetable and slice limits. */
if (size <= PAGE_SIZE || !is_power_of_2(size))
--
2.48.1
