On Fri, Oct 18, 2019 at 03:49:42PM +0300, Jarkko Sakkinen wrote: > On Wed, Oct 16, 2019 at 11:37:45AM -0700, Sean Christopherson wrote: > > Track the free page count on a per EPC section basis so that the value > > is properly protected by the section's spinlock. > > > > As was pointed out when the change was proposed[*], using a global > > non-atomic counter to track the number of free EPC pages is not safe. > > The order of non-atomic reads and writes are not guaranteed, i.e. > > concurrent RMW operats can write stale data. This causes a variety > > of bad behavior, e.g. livelocks because the free page count wraps and > > causes the swap thread to stop reclaiming. > > > > Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx> > > What is the reason not change it just to atomic? The purpose of separate sections is to avoid bouncing locks and whatnot across packages. Adding a global atomic to the hotpath defeats that purpose. > For debugging the global is useful because it could be exposed > as a sysfs file. Can't the sysfs read helper aggregate the info? > > --- > > arch/x86/kernel/cpu/sgx/main.c | 11 +++++------ > > arch/x86/kernel/cpu/sgx/reclaim.c | 4 ++-- > > arch/x86/kernel/cpu/sgx/sgx.h | 18 +++++++++++++++++- > > 3 files changed, 24 insertions(+), 9 deletions(-) > > > > diff --git a/arch/x86/kernel/cpu/sgx/main.c b/arch/x86/kernel/cpu/sgx/main.c > > index 6311aef10ec4..efbb52e4ecad 100644 > > --- a/arch/x86/kernel/cpu/sgx/main.c > > +++ b/arch/x86/kernel/cpu/sgx/main.c > > @@ -13,18 +13,17 @@ > > > > struct sgx_epc_section sgx_epc_sections[SGX_MAX_EPC_SECTIONS]; > > int sgx_nr_epc_sections; > > -unsigned long sgx_nr_free_pages; > > > > static struct sgx_epc_page *__sgx_try_alloc_page(struct sgx_epc_section *section) > > { > > struct sgx_epc_page *page; > > > > - if (list_empty(§ion->page_list)) > > + if (!section->free_cnt) > > return NULL; > > Why this check needs to be changed? I was reverting to the previous behavior. > > > > page = list_first_entry(§ion->page_list, struct sgx_epc_page, list); > > list_del_init(&page->list); > > - sgx_nr_free_pages--; > > + section->free_cnt--; > > return page; > > } > > > > @@ -97,7 +96,7 @@ struct sgx_epc_page *sgx_alloc_page(void *owner, bool reclaim) > > schedule(); > > } > > > > - if (sgx_nr_free_pages < SGX_NR_LOW_PAGES) > > + if (!sgx_at_least_N_free_pages(SGX_NR_LOW_PAGES)) > > wake_up(&ksgxswapd_waitq); > > > > return entry; > > @@ -131,7 +130,7 @@ void __sgx_free_page(struct sgx_epc_page *page) > > > > spin_lock(§ion->lock); > > list_add_tail(&page->list, §ion->page_list); > > - sgx_nr_free_pages++; > > + section->free_cnt++; > > spin_unlock(§ion->lock); > > > > } > > @@ -218,7 +217,7 @@ static bool __init sgx_alloc_epc_section(u64 addr, u64 size, > > list_add_tail(&page->list, §ion->unsanitized_page_list); > > } > > > > - sgx_nr_free_pages += nr_pages; > > + section->free_cnt = nr_pages; > > > > return true; > > > > diff --git a/arch/x86/kernel/cpu/sgx/reclaim.c b/arch/x86/kernel/cpu/sgx/reclaim.c > > index 3f183dd0e653..8619141f4bed 100644 > > --- a/arch/x86/kernel/cpu/sgx/reclaim.c > > +++ b/arch/x86/kernel/cpu/sgx/reclaim.c > > @@ -68,7 +68,7 @@ static void sgx_sanitize_section(struct sgx_epc_section *section) > > > > static inline bool sgx_should_reclaim(void) > > { > > - return sgx_nr_free_pages < SGX_NR_HIGH_PAGES && > > + return !sgx_at_least_N_free_pages(SGX_NR_HIGH_PAGES) && > > !list_empty(&sgx_active_page_list); > > } > > > > @@ -430,7 +430,7 @@ void sgx_reclaim_pages(void) > > section = sgx_epc_section(epc_page); > > spin_lock(§ion->lock); > > list_add_tail(&epc_page->list, §ion->page_list); > > - sgx_nr_free_pages++; > > + section->free_cnt++; > > spin_unlock(§ion->lock); > > } > > } > > diff --git a/arch/x86/kernel/cpu/sgx/sgx.h b/arch/x86/kernel/cpu/sgx/sgx.h > > index 87e375e8c25e..c7f0277299f6 100644 > > --- a/arch/x86/kernel/cpu/sgx/sgx.h > > +++ b/arch/x86/kernel/cpu/sgx/sgx.h > > @@ -30,6 +30,7 @@ struct sgx_epc_page { > > struct sgx_epc_section { > > unsigned long pa; > > void *va; > > + unsigned long free_cnt; > > struct list_head page_list; > > struct list_head unsanitized_page_list; > > spinlock_t lock; > > @@ -73,12 +74,27 @@ static inline void *sgx_epc_addr(struct sgx_epc_page *page) > > #define SGX_NR_HIGH_PAGES 64 > > > > extern int sgx_nr_epc_sections; > > -extern unsigned long sgx_nr_free_pages; > > extern struct task_struct *ksgxswapd_tsk; > > extern struct wait_queue_head(ksgxswapd_waitq); > > extern struct list_head sgx_active_page_list; > > extern spinlock_t sgx_active_page_list_lock; > > > > +static inline bool sgx_at_least_N_free_pages(unsigned long threshold) > > There is an upper case letter in the function name and name is also > weird overally. Ya, not a fan of the name either. > > +{ > > + struct sgx_epc_section *section; > > + unsigned long free_cnt = 0; > > + int i; > > + > > + for (i = 0; i < sgx_nr_epc_sections; i++) { > > + section = &sgx_epc_sections[i]; > > + free_cnt += section->free_cnt; > > + if (free_cnt >= threshold) > > + return true; > > + } > > + > > + return false; > > +} > > The complexity does not pay here. Better to revert instead back to this > if required: I'd prefer to optimize for the happy case, even if it's minor. But I'm fine going with the below code. > unsigned long sgx_calc_free_cnt(void) > { > struct sgx_epc_section *section; > unsigned long free_cnt = 0; > int i; > > for (i = 0; i < sgx_nr_epc_sections; i++) { > section = &sgx_epc_sections[i]; > free_cnt += section->free_cnt; > } > > return free_cnt; > } > > /Jarkko