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Message-ID: <dd0c92b47496be5f59cc416c6e135ef573184c66.camel@linux.intel.com>
Date: Thu, 08 Dec 2022 10:13:36 -0800
From: Kristen Carlson Accardi <kristen@...ux.intel.com>
To: Jarkko Sakkinen <jarkko@...nel.org>
Cc: dave.hansen@...ux.intel.com, tj@...nel.org,
linux-kernel@...r.kernel.org, linux-sgx@...r.kernel.org,
cgroups@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
zhiquan1.li@...el.com, Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH v2 06/18] x86/sgx: Introduce RECLAIM_IN_PROGRESS flag
for EPC pages
On Thu, 2022-12-08 at 15:46 +0000, Jarkko Sakkinen wrote:
> On Fri, Dec 02, 2022 at 10:36:42AM -0800, Kristen Carlson Accardi
> wrote:
> > From: Sean Christopherson <sean.j.christopherson@...el.com>
> >
> > When selecting pages to be reclaimed from the page pool
> > (sgx_global_lru),
> > the list of reclaimable pages is walked, and any page that is both
> > reclaimable and not in the process of being freed is added to a
> > list of
> > potential candidates to be reclaimed. After that, this separate
> > list is
> > further examined and may or may not ultimately be reclaimed. In
> > order
> > to prevent this page from being removed from the sgx_epc_lru_lists
> > struct in a separate thread by sgx_drop_epc_page(), keep track of
> > whether the EPC page is in the middle of being reclaimed with
> > the addtion of a RECLAIM_IN_PROGRESS flag, and do not delete the
> > page
> > off the LRU in sgx_drop_epc_page() if it has not yet finished being
> > reclaimed.
> >
> > Signed-off-by: Sean Christopherson
> > <sean.j.christopherson@...el.com>
> > Signed-off-by: Kristen Carlson Accardi <kristen@...ux.intel.com>
> > Cc: Sean Christopherson <seanjc@...gle.com>
> > ---
> > arch/x86/kernel/cpu/sgx/main.c | 15 ++++++++++-----
> > arch/x86/kernel/cpu/sgx/sgx.h | 2 ++
> > 2 files changed, 12 insertions(+), 5 deletions(-)
> >
> > diff --git a/arch/x86/kernel/cpu/sgx/main.c
> > b/arch/x86/kernel/cpu/sgx/main.c
> > index ecd7f8e704cc..bad72498b0a7 100644
> > --- a/arch/x86/kernel/cpu/sgx/main.c
> > +++ b/arch/x86/kernel/cpu/sgx/main.c
> > @@ -305,13 +305,15 @@ static void __sgx_reclaim_pages(void)
> >
> > encl_page = epc_page->encl_owner;
> >
> > - if (kref_get_unless_zero(&encl_page->encl-
> > >refcount) != 0)
> > + if (kref_get_unless_zero(&encl_page->encl-
> > >refcount) != 0) {
> > + epc_page->flags |=
> > SGX_EPC_PAGE_RECLAIM_IN_PROGRESS;
> > chunk[cnt++] = epc_page;
> > - else
> > + } else {
> > /* The owner is freeing the page. No need
> > to add the
> > * page back to the list of reclaimable
> > pages.
> > */
> > epc_page->flags &=
> > ~SGX_EPC_PAGE_RECLAIMER_TRACKED;
> > + }
> > }
> > spin_unlock(&sgx_global_lru.lock);
> >
> > @@ -337,6 +339,7 @@ static void __sgx_reclaim_pages(void)
> >
> > skip:
> > spin_lock(&sgx_global_lru.lock);
> > + epc_page->flags &=
> > ~SGX_EPC_PAGE_RECLAIM_IN_PROGRESS;
> > sgx_epc_push_reclaimable(&sgx_global_lru,
> > epc_page);
> > spin_unlock(&sgx_global_lru.lock);
> >
> > @@ -360,7 +363,8 @@ static void __sgx_reclaim_pages(void)
> > sgx_reclaimer_write(epc_page, &backing[i]);
> >
> > kref_put(&encl_page->encl->refcount,
> > sgx_encl_release);
> > - epc_page->flags &= ~SGX_EPC_PAGE_RECLAIMER_TRACKED;
> > + epc_page->flags &= ~(SGX_EPC_PAGE_RECLAIMER_TRACKED
> > |
> > +
> > SGX_EPC_PAGE_RECLAIM_IN_PROGRESS);
> >
> > sgx_free_epc_page(epc_page);
> > }
> > @@ -508,7 +512,8 @@ struct sgx_epc_page *__sgx_alloc_epc_page(void)
> > void sgx_record_epc_page(struct sgx_epc_page *page, unsigned long
> > flags)
> > {
> > spin_lock(&sgx_global_lru.lock);
> > - WARN_ON(page->flags & SGX_EPC_PAGE_RECLAIMER_TRACKED);
> > + WARN_ON(page->flags & (SGX_EPC_PAGE_RECLAIMER_TRACKED |
> > + SGX_EPC_PAGE_RECLAIM_IN_PROGRESS));
> > page->flags |= flags;
> > if (flags & SGX_EPC_PAGE_RECLAIMER_TRACKED)
> > sgx_epc_push_reclaimable(&sgx_global_lru, page);
> > @@ -532,7 +537,7 @@ int sgx_drop_epc_page(struct sgx_epc_page
> > *page)
> > spin_lock(&sgx_global_lru.lock);
> > if (page->flags & SGX_EPC_PAGE_RECLAIMER_TRACKED) {
> > /* The page is being reclaimed. */
> > - if (list_empty(&page->list)) {
> > + if (page->flags & SGX_EPC_PAGE_RECLAIM_IN_PROGRESS)
> > {
> > spin_unlock(&sgx_global_lru.lock);
> > return -EBUSY;
> > }
> > diff --git a/arch/x86/kernel/cpu/sgx/sgx.h
> > b/arch/x86/kernel/cpu/sgx/sgx.h
> > index ba4338b7303f..37d66bc6ca27 100644
> > --- a/arch/x86/kernel/cpu/sgx/sgx.h
> > +++ b/arch/x86/kernel/cpu/sgx/sgx.h
> > @@ -30,6 +30,8 @@
> > #define SGX_EPC_PAGE_IS_FREE BIT(1)
> > /* Pages allocated for KVM guest */
> > #define SGX_EPC_PAGE_KVM_GUEST BIT(2)
> > +/* page flag to indicate reclaim is in progress */
> > +#define SGX_EPC_PAGE_RECLAIM_IN_PROGRESS BIT(3)
> >
> > struct sgx_epc_page {
> > unsigned int section;
> > --
> > 2.38.1
> >
>
> I would create:
>
> enum sgx_epc_state {
> SGX_EPC_STATE_READY = 0,
> SGX_EPC_STATE_RECLAIMER_TRACKED = 1,
> SGX_EPC_STATE_RECLAIM_IN_PROGRESS = 2,
> };
>
> I.e. not a bitmask because page should have only one state at
> a time for any of this to make any sense. We have an FSM,
> right?
>
I can experiment with it and see if it can work for the flags to
represent a state machine. They don't work like that right now
obviously, since you can have both RECLAIMER_TRACKED and
RECLAIM_IN_PROGRESS set at the same time.
> And then allocate 2 upper bits to store this information from
> flags.
>
> And probably would make sense to have inline helper functions
> to setting and getting the state that does the bitshifting and
> masking shenanigangs.
>
> This would be a patch prepending this.
>
> In this patch you should then describe in the context of FSM
> how EPC page moves between these states. With that knowledge
> we can then reflect the actual code change.
>
> The point is not to get right but more like a mindset that we
> can discuss right or wrong in thee first place so just do your
> best but don't stress too much.
>
> BR, Jarkko
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