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author | David Hildenbrand <david@redhat.com> | 2024-04-11 19:14:41 +0300 |
---|---|---|
committer | Greg Kroah-Hartman <gregkh@linuxfoundation.org> | 2024-05-30 10:49:00 +0300 |
commit | febb5f8f33d5bb1088b78ab65559d1e5839933ba (patch) | |
tree | 3c1e9ca1b0b02463431ef2ce1de253d1e35894e5 /arch/s390/include/asm | |
parent | 5c8323a24d6e3c42b99518b84ae6fcc5341f8fbe (diff) | |
download | linux-febb5f8f33d5bb1088b78ab65559d1e5839933ba.tar.xz |
s390/mm: Re-enable the shared zeropage for !PV and !skeys KVM guests
[ Upstream commit 06201e00ee3e4beacac48aab2b83eff64ebf0bc0 ]
commit fa41ba0d08de ("s390/mm: avoid empty zero pages for KVM guests to
avoid postcopy hangs") introduced an undesired side effect when combined
with memory ballooning and VM migration: memory part of the inflated
memory balloon will consume memory.
Assuming we have a 100GiB VM and inflated the balloon to 40GiB. Our VM
will consume ~60GiB of memory. If we now trigger a VM migration,
hypervisors like QEMU will read all VM memory. As s390x does not support
the shared zeropage, we'll end up allocating for all previously-inflated
memory part of the memory balloon: 50 GiB. So we might easily
(unexpectedly) crash the VM on the migration source.
Even worse, hypervisors like QEMU optimize for zeropage migration to not
consume memory on the migration destination: when migrating a
"page full of zeroes", on the migration destination they check whether the
target memory is already zero (by reading the destination memory) and avoid
writing to the memory to not allocate memory: however, s390x will also
allocate memory here, implying that also on the migration destination, we
will end up allocating all previously-inflated memory part of the memory
balloon.
This is especially bad if actual memory overcommit was not desired, when
memory ballooning is used for dynamic VM memory resizing, setting aside
some memory during boot that can be added later on demand. Alternatives
like virtio-mem that would avoid this issue are not yet available on
s390x.
There could be ways to optimize some cases in user space: before reading
memory in an anonymous private mapping on the migration source, check via
/proc/self/pagemap if anything is already populated. Similarly check on
the migration destination before reading. While that would avoid
populating tables full of shared zeropages on all architectures, it's
harder to get right and performant, and requires user space changes.
Further, with posctopy live migration we must place a page, so there,
"avoid touching memory to avoid allocating memory" is not really
possible. (Note that a previously we would have falsely inserted
shared zeropages into processes using UFFDIO_ZEROPAGE where
mm_forbids_zeropage() would have actually forbidden it)
PV is currently incompatible with memory ballooning, and in the common
case, KVM guests don't make use of storage keys. Instead of zapping
zeropages when enabling storage keys / PV, that turned out to be
problematic in the past, let's do exactly the same we do with KSM pages:
trigger unsharing faults to replace the shared zeropages by proper
anonymous folios.
What about added latency when enabling storage kes? Having a lot of
zeropages in applicable environments (PV, legacy guests, unittests) is
unexpected. Further, KSM could today already unshare the zeropages
and unmerging KSM pages when enabling storage kets would unshare the
KSM-placed zeropages in the same way, resulting in the same latency.
[ agordeev: Fixed sparse and checkpatch complaints and error handling ]
Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@linux.ibm.com>
Fixes: fa41ba0d08de ("s390/mm: avoid empty zero pages for KVM guests to avoid postcopy hangs")
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20240411161441.910170-3-david@redhat.com
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Diffstat (limited to 'arch/s390/include/asm')
-rw-r--r-- | arch/s390/include/asm/gmap.h | 2 | ||||
-rw-r--r-- | arch/s390/include/asm/mmu.h | 5 | ||||
-rw-r--r-- | arch/s390/include/asm/mmu_context.h | 1 | ||||
-rw-r--r-- | arch/s390/include/asm/pgtable.h | 16 |
4 files changed, 20 insertions, 4 deletions
diff --git a/arch/s390/include/asm/gmap.h b/arch/s390/include/asm/gmap.h index 5cc46e0dde62..9725586f4259 100644 --- a/arch/s390/include/asm/gmap.h +++ b/arch/s390/include/asm/gmap.h @@ -146,7 +146,7 @@ int gmap_mprotect_notify(struct gmap *, unsigned long start, void gmap_sync_dirty_log_pmd(struct gmap *gmap, unsigned long dirty_bitmap[4], unsigned long gaddr, unsigned long vmaddr); -int gmap_mark_unmergeable(void); +int s390_disable_cow_sharing(void); void s390_unlist_old_asce(struct gmap *gmap); int s390_replace_asce(struct gmap *gmap); void s390_uv_destroy_pfns(unsigned long count, unsigned long *pfns); diff --git a/arch/s390/include/asm/mmu.h b/arch/s390/include/asm/mmu.h index bb1b4bef1878..4c2dc7abc285 100644 --- a/arch/s390/include/asm/mmu.h +++ b/arch/s390/include/asm/mmu.h @@ -32,6 +32,11 @@ typedef struct { unsigned int uses_skeys:1; /* The mmu context uses CMM. */ unsigned int uses_cmm:1; + /* + * The mmu context allows COW-sharing of memory pages (KSM, zeropage). + * Note that COW-sharing during fork() is currently always allowed. + */ + unsigned int allow_cow_sharing:1; /* The gmaps associated with this context are allowed to use huge pages. */ unsigned int allow_gmap_hpage_1m:1; } mm_context_t; diff --git a/arch/s390/include/asm/mmu_context.h b/arch/s390/include/asm/mmu_context.h index 929af18b0908..a7789a9f6218 100644 --- a/arch/s390/include/asm/mmu_context.h +++ b/arch/s390/include/asm/mmu_context.h @@ -35,6 +35,7 @@ static inline int init_new_context(struct task_struct *tsk, mm->context.has_pgste = 0; mm->context.uses_skeys = 0; mm->context.uses_cmm = 0; + mm->context.allow_cow_sharing = 1; mm->context.allow_gmap_hpage_1m = 0; #endif switch (mm->context.asce_limit) { diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h index 12a7b8678925..0a7055518ba2 100644 --- a/arch/s390/include/asm/pgtable.h +++ b/arch/s390/include/asm/pgtable.h @@ -566,10 +566,20 @@ static inline pud_t set_pud_bit(pud_t pud, pgprot_t prot) } /* - * In the case that a guest uses storage keys - * faults should no longer be backed by zero pages + * As soon as the guest uses storage keys or enables PV, we deduplicate all + * mapped shared zeropages and prevent new shared zeropages from getting + * mapped. */ -#define mm_forbids_zeropage mm_has_pgste +#define mm_forbids_zeropage mm_forbids_zeropage +static inline int mm_forbids_zeropage(struct mm_struct *mm) +{ +#ifdef CONFIG_PGSTE + if (!mm->context.allow_cow_sharing) + return 1; +#endif + return 0; +} + static inline int mm_uses_skeys(struct mm_struct *mm) { #ifdef CONFIG_PGSTE |