Re: [PATCH 5/6] nfsd: add support for freeing unused session-DRC slots

Chuck Lever III
2024-12-02T16:11:37Z

> On Nov 21, 2024, at 5:29 PM, Chuck Lever III <chuck.lever@oracle.com> wrote:
>
>
>
>> On Nov 21, 2024, at 4:47 PM, NeilBrown <neilb@suse.de> wrote:
>>
>> On Wed, 20 Nov 2024, Chuck Lever wrote:
>>> On Wed, Nov 20, 2024 at 09:35:00AM +1100, NeilBrown wrote:
>>>> On Wed, 20 Nov 2024, Chuck Lever wrote:
>>>>> On Tue, Nov 19, 2024 at 11:41:32AM +1100, NeilBrown wrote:
>>>>>> Reducing the number of slots in the session slot table requires
>>>>>> confirmation from the client. This patch adds reduce_session_slots()
>>>>>> which starts the process of getting confirmation, but never calls it.
>>>>>> That will come in a later patch.
>>>>>>
>>>>>> Before we can free a slot we need to confirm that the client won't try
>>>>>> to use it again. This involves returning a lower cr_maxrequests in a
>>>>>> SEQUENCE reply and then seeing a ca_maxrequests on the same slot which
>>>>>> is not larger than we limit we are trying to impose. So for each slot
>>>>>> we need to remember that we have sent a reduced cr_maxrequests.
>>>>>>
>>>>>> To achieve this we introduce a concept of request "generations". Each
>>>>>> time we decide to reduce cr_maxrequests we increment the generation
>>>>>> number, and record this when we return the lower cr_maxrequests to the
>>>>>> client. When a slot with the current generation reports a low
>>>>>> ca_maxrequests, we commit to that level and free extra slots.
>>>>>>
>>>>>> We use an 8 bit generation number (64 seems wasteful) and if it cycles
>>>>>> we iterate all slots and reset the generation number to avoid false matches.
>>>>>>
>>>>>> When we free a slot we store the seqid in the slot pointer so that it can
>>>>>> be restored when we reactivate the slot. The RFC can be read as
>>>>>> suggesting that the slot number could restart from one after a slot is
>>>>>> retired and reactivated, but also suggests that retiring slots is not
>>>>>> required. So when we reactive a slot we accept with the next seqid in
>>>>>> sequence, or 1.
>>>>>>
>>>>>> When decoding sa_highest_slotid into maxslots we need to add 1 - this
>>>>>> matches how it is encoded for the reply.
>>>>>>
>>>>>> Signed-off-by: NeilBrown <neilb@suse.de>
>>>>>> ---
>>>>>> fs/nfsd/nfs4state.c | 81 ++++++++++++++++++++++++++++++++++++++-------
>>>>>> fs/nfsd/nfs4xdr.c | 5 +--
>>>>>> fs/nfsd/state.h | 4 +++
>>>>>> fs/nfsd/xdr4.h | 2 --
>>>>>> 4 files changed, 76 insertions(+), 16 deletions(-)
>>>>>>
>>>>>> diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
>>>>>> index fb522165b376..0625b0aec6b8 100644
>>>>>> --- a/fs/nfsd/nfs4state.c
>>>>>> +++ b/fs/nfsd/nfs4state.c
>>>>>> @@ -1910,17 +1910,55 @@ gen_sessionid(struct nfsd4_session *ses)
>>>>>> #define NFSD_MIN_HDR_SEQ_SZ (24 + 12 + 44)
>>>>>>
>>>>>> static void
>>>>>> -free_session_slots(struct nfsd4_session *ses)
>>>>>> +free_session_slots(struct nfsd4_session *ses, int from)
>>>>>> {
>>>>>> int i;
>>>>>>
>>>>>> - for (i = 0; i < ses->se_fchannel.maxreqs; i++) {
>>>>>> + if (from >= ses->se_fchannel.maxreqs)
>>>>>> + return;
>>>>>> +
>>>>>> + for (i = from; i < ses->se_fchannel.maxreqs; i++) {
>>>>>> struct nfsd4_slot *slot = xa_load(&ses->se_slots, i);
>>>>>>
>>>>>> - xa_erase(&ses->se_slots, i);
>>>>>> + /*
>>>>>> + * Save the seqid in case we reactivate this slot.
>>>>>> + * This will never require a memory allocation so GFP
>>>>>> + * flag is irrelevant
>>>>>> + */
>>>>>> + xa_store(&ses->se_slots, i, xa_mk_value(slot->sl_seqid),
>>>>>> + GFP_ATOMIC);
>>>>>
>>>>> Again... ATOMIC is probably not what we want here, even if it is
>>>>> only documentary.
>>>>
>>>> Why not? It might be called under a spinlock so GFP_KERNEL might trigger
>>>> a warning.
>>>
>>> I find using GFP_ATOMIC here to be confusing -- it requests
>>> allocation from special memory reserves and is to be used in
>>> situations where allocation might result in system failure. That is
>>> clearly not the case here, and the resulting memory allocation might
>>> be long-lived.
>>
>> Would you be comfortable with GFP_NOWAIT which leaves out __GFP_HIGH ??
>
> I will be comfortable when I hear back from Matthew and Liam.
>
> :-)
>
>
>>> I see the comment that says memory won't actually be allocated. I'm
>>> not sure that's the way xa_store() works, however.
>>
>> xarray.rst says:
>>
>> The xa_store(), xa_cmpxchg(), xa_alloc(),
>> xa_reserve() and xa_insert() functions take a gfp_t
>> parameter in case the XArray needs to allocate memory to store this entry.
>> If the entry is being deleted, no memory allocation needs to be performed,
>> and the GFP flags specified will be ignored.`
>>
>> The particular context is that a normal pointer is currently stored a
>> the given index, and we are replacing that with a number. The above
>> doesn't explicitly say that won't require a memory allocation, but I
>> think it is reasonable to say it won't need "to allocate memory to store
>> this entry" - as an entry is already stored - so allocation should not
>> be needed.
>
> xa_mk_value() converts a number to a pointer, and xa_store
> stores that pointer.
>
> I suspect that xa_store() is allowed to rebalance the
> xarray's internal data structures, and that could result
> in memory release or allocation. That's why a GFP flag is
> one of the arguments.

Matthew says the xa_store() is guaranteed not to do a memory
allocation in this case. However, they prefer an annotation
of the call site with a "0" GFP argument to show that the
allocation flags are not relevant.

Does this:

xa_store(&ses->se_slots, i, xa_mk_value(slot->sl_seqid), 0);

work for you?

--
Chuck Lever