Thanks for the review!
> On Jun 29, 2017, at 4:20 PM, J. Bruce Fields <bfields@xxxxxxxxxxxx> wrote:
>
> I'm confused by this one:
>
> On Fri, Jun 23, 2017 at 05:18:16PM -0400, Chuck Lever wrote:
>> svcrdma needs 259 pages for 1MB NFSv4.0 READ requests:
>>
>> - 1 page for the transport header and head iovec
>> - 256 pages for the data payload
>> - 1 page for the trailing GETATTR request (since NFSD XDR decoding
>> does not look for a tail iovec, the GETATTR is stuck at the end
>> of the rqstp->rq_arg.pages list)
>> - 1 page for building the reply xdr_buf
>>
>> Note that RPCSVC_MAXPAGES is defined as:
>>
>> ((RPCSVC_MAXPAYLOAD+PAGE_SIZE-1)/PAGE_SIZE + 2 + 1)
>>
>> I don't understand why the "+PAGE_SIZE-1" is in there, because
>> division will always round the result down to
>>
>> (RPCSVC_MAXPAYLOAD / PAGE_SIZE + 2 + 1)
>
> I think you're assuming that RPCSVC_MAXPAYLOAD is a multiple of
> PAGE_SIZE. Maybe that's true in all cases (I haven't checked), but it
> seems harmless to handle the case where it's not.
include/linux/sunrpc/svc.h says:
126 * Maximum payload size supported by a kernel RPC server.
127 * This is use to determine the max number of pages nfsd is
128 * willing to return in a single READ operation.
129 *
130 * These happen to all be powers of 2, which is not strictly
131 * necessary but helps enforce the real limitation, which is
132 * that they should be multiples of PAGE_SIZE.
Note that the same calculation in svc_alloc_args() assumes
that serv->sv_max_mesg is a multiple of PAGE_SIZE:
pages = (serv->sv_max_mesg + PAGE_SIZE) / PAGE_SIZE;
It's confusing that the value of RPCSVC_MAXPAGES is not computed
the same way as "pages" is.
>> Let's remove the "-1" to get 260 pages maximum.
>
> Why? (I'm not necessarily opposed, I just missed the explanation.)
The top of the patch description explains why I need 259 pages.
MAX_PAGES has to be 260 in order for this check in svc_alloc_args()
if (pages >= RPCSVC_MAXPAGES) {
/* use as many pages as possible */
pages = RPCSVC_MAXPAGES - 1;
to allow 260 elements of the array to be filled in.
> --b.
>
>> Then svc_alloc_args() needs to ask for pages according to this
>> same formula, otherwise it will never allocate enough.
>>
>> Signed-off-by: Chuck Lever <chuck.lever@xxxxxxxxxx>
>> ---
>> include/linux/sunrpc/svc.h | 3 +--
>> net/sunrpc/svc_xprt.c | 8 +++++---
>> 2 files changed, 6 insertions(+), 5 deletions(-)
>>
>> diff --git a/include/linux/sunrpc/svc.h b/include/linux/sunrpc/svc.h
>> index 11cef5a..35c274f 100644
>> --- a/include/linux/sunrpc/svc.h
>> +++ b/include/linux/sunrpc/svc.h
>> @@ -176,8 +176,7 @@ static inline void svc_get(struct svc_serv *serv)
>> * We using ->sendfile to return read data, we might need one extra page
>> * if the request is not page-aligned. So add another '1'.
>> */
>> -#define RPCSVC_MAXPAGES ((RPCSVC_MAXPAYLOAD+PAGE_SIZE-1)/PAGE_SIZE \
>> - + 2 + 1)
>> +#define RPCSVC_MAXPAGES ((RPCSVC_MAXPAYLOAD + PAGE_SIZE) / PAGE_SIZE + 2 + 1)
>>
>> static inline u32 svc_getnl(struct kvec *iov)
>> {
>> diff --git a/net/sunrpc/svc_xprt.c b/net/sunrpc/svc_xprt.c
>> index 7bfe1fb..9bd484d 100644
>> --- a/net/sunrpc/svc_xprt.c
>> +++ b/net/sunrpc/svc_xprt.c
>> @@ -659,11 +659,13 @@ static int svc_alloc_arg(struct svc_rqst *rqstp)
>> int i;
>>
>> /* now allocate needed pages. If we get a failure, sleep briefly */
>> - pages = (serv->sv_max_mesg + PAGE_SIZE) / PAGE_SIZE;
>> - WARN_ON_ONCE(pages >= RPCSVC_MAXPAGES);
>> - if (pages >= RPCSVC_MAXPAGES)
>> + pages = (serv->sv_max_mesg + (2 * PAGE_SIZE)) >> PAGE_SHIFT;
>> + if (pages >= RPCSVC_MAXPAGES) {
>> + pr_warn_once("svc: warning: pages=%u >= RPCSVC_MAXPAGES=%lu\n",
>> + pages, RPCSVC_MAXPAGES);
>> /* use as many pages as possible */
>> pages = RPCSVC_MAXPAGES - 1;
>> + }
>> for (i = 0; i < pages ; i++)
>> while (rqstp->rq_pages[i] == NULL) {
>> struct page *p = alloc_page(GFP_KERNEL);
> --
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--
Chuck Lever
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