Re: [PATCH v3] ndctl, check: Ensure mmap of BTT sections work with 64K page-sizes

[Vaibhav Jain <vaibhav@linux.ibm.com>]

Thanks for reviewing this patch Jeff.

Jeff Moyer <jmoyer@redhat.com> writes:

> Vaibhav Jain <vaibhav@linux.ibm.com> writes:
>
>> On PPC64 which uses a 64K page-size, ndtl-check command fails on a BTT
>> namespace with following error:
>>
>> $ sudo ndctl check-namespace namespace0.0 -vv
>>   namespace0.0: namespace_check: checking namespace0.0
>>   namespace0.0: btt_discover_arenas: found 1 BTT arena
>>   namespace0.0: btt_create_mappings: mmap arena[0].info [sz = 0x1000, off = 0x1000] failed: Invalid argument
>>   error checking namespaces: Invalid argument
>>   checked 0 namespaces
>>
>> An error happens when btt_create_mappings() tries to mmap the sections
>> of the BTT device which are usually 4K offset aligned. However the
>> mmap() syscall expects the 'offset' argument to be in multiples of
>> page-size, hence it returns EINVAL causing the btt_create_mappings()
>> to error out.
>>
>> As a fix for the issue this patch proposes addition of two new
>> functions to 'check.c' namely btt_mmap/btt_unmap that can be used to
>> map/unmap parts of BTT device to ndctl process address-space. The
>> functions tweaks the requested 'offset' argument to mmap() ensuring
>> that its page-size aligned and then fix-ups the returned pointer such
>> that it points to the requested offset within mmapped region.
>>
>> With these newly introduced functions the patch updates the call-sites
>> in 'check.c' to use these functions instead of mmap/unmap syscalls.
>> Also since btt_mmap returns NULL if mmap operation fails, the
>> error check call-sites can be made against NULL instead of MAP_FAILED.
>>
>> Reported-by: Harish Sriram <harish@linux.ibm.com>
>> Signed-off-by: Vaibhav Jain <vaibhav@linux.ibm.com>
>> ---
>> Changelog:
>> v3:
>> * Fixed a log string that was splitted across multiple lines [Vishal]
>>
>> v2:
>> * Updated patch description to include canonical email address of bug
>>   reporter. [Vishal]
>> * Improved the comment describing function btt_mmap() in 'check.c'
>>   [Vishal]
>> * Simplified the argument list for btt_mmap() to just accept bttc,
>>   length and offset. Other arguments for mmap() are derived from these
>>   args. [Vishal]
>> * Renamed 'shift' variable in btt_mmap()/unmap() to 'page_offset'
>>   [Vishal]
>> * Improved the dbg() messages logged inside
>>   btt_mmap()/unmap(). [Vishal]
>> * Return NULL from btt_mmap() in case of an error. [Vishal]
>> * Changed the all sites to btt_mmap() to perform error check against
>>   NULL return value instead of MAP_FAILED. [Vishal]
>> ---
>>  ndctl/check.c | 93 +++++++++++++++++++++++++++++++++++++--------------
>>  1 file changed, 67 insertions(+), 26 deletions(-)
>>
>> diff --git a/ndctl/check.c b/ndctl/check.c
>> index 8a7125053865..5969012eba84 100644
>> --- a/ndctl/check.c
>> +++ b/ndctl/check.c
>> @@ -907,59 +907,100 @@ static int btt_discover_arenas(struct btt_chk *bttc)
>>  	return ret;
>>  }
>>  
>> -static int btt_create_mappings(struct btt_chk *bttc)
>> +/*
>> + * Wrap call to mmap(2) to work with btt device offsets that are not aligned
>> + * to system page boundary. It works by rounding down the requested offset
>> + * to sys_page_size when calling mmap(2) and then returning a fixed-up pointer
>> + * to the correct offset in the mmaped region.
>> + */
>> +static void *btt_mmap(struct btt_chk *bttc, size_t length, off_t offset)
>>  {
>> -	struct arena_info *a;
>> -	int mmap_flags;
>> -	int i;
>> +	off_t page_offset;
>> +	int prot_flags;
>> +	uint8_t *addr;
>>  
>>  	if (!bttc->opts->repair)
>> -		mmap_flags = PROT_READ;
>> +		prot_flags = PROT_READ;
>>  	else
>> -		mmap_flags = PROT_READ|PROT_WRITE;
>> +		prot_flags = PROT_READ|PROT_WRITE;
>> +
>> +	/* Calculate the page_offset from the system page boundary */
>> +	page_offset = offset - rounddown(offset, bttc->sys_page_size);
>> +
>> +	/* Update the offset and length with the page_offset calculated above */
>> +	offset -= page_offset;
>> +	length += page_offset;
>
> Don't you have to ensure that the length is also a multiple of the
> system page size?
>
> -Jeff

No, as the BTT info header is 4K in size which isnt in multiple of page
size on PPC64 where PAGE_SIZE == 64K.

Also I see 'do_mmap()' in kernel always rounding up the 'length' to
PAGE_SIZE. So any unaligned value for 'length' arg will be handled by the
kernel.

Finally mmap(2) doesn't put any constraint on the 'length' argument to
mmap except it should > 0. The 'offset' arg on other hand has a
constraint which needs to be in multiple of PAGE_SIZE.

>
>> +
>> +	addr = mmap(NULL, length, prot_flags, MAP_SHARED, bttc->fd, offset);
>> +
>> +	/* If needed fixup the return pointer to correct offset requested */
>> +	if (addr != MAP_FAILED)
>> +		addr += page_offset;
>> +
>> +	dbg(bttc, "addr = %p, length = %#lx, offset = %#lx, page_offset = %#lx\n",
>> +	    (void *) addr, length, offset, page_offset);
>> +
>> +	return addr == MAP_FAILED ? NULL : addr;
>> +}
>> +
>> +static void btt_unmap(struct btt_chk *bttc, void *ptr, size_t length)
>> +{
>> +	off_t page_offset;
>> +	uintptr_t addr = (uintptr_t) ptr;
>> +
>> +	/* Calculate the page_offset from system page boundary */
>> +	page_offset = addr - rounddown(addr, bttc->sys_page_size);
>> +
>> +	addr -= page_offset;
>> +	length += page_offset;
>> +
>> +	munmap((void *) addr, length);
>> +	dbg(bttc, "addr = %p, length = %#lx, page_offset = %#lx\n",
>> +	    (void *) addr, length, page_offset);
>> +}
>> +
>> +static int btt_create_mappings(struct btt_chk *bttc)
>> +{
>> +	struct arena_info *a;
>> +	int i;
>>  
>>  	for (i = 0; i < bttc->num_arenas; i++) {
>>  		a = &bttc->arena[i];
>>  		a->map.info_len = BTT_INFO_SIZE;
>> -		a->map.info = mmap(NULL, a->map.info_len, mmap_flags,
>> -			MAP_SHARED, bttc->fd, a->infooff);
>> -		if (a->map.info == MAP_FAILED) {
>> +		a->map.info = btt_mmap(bttc, a->map.info_len, a->infooff);
>> +		if (!a->map.info) {
>>  			err(bttc, "mmap arena[%d].info [sz = %#lx, off = %#lx] failed: %s\n",
>>  				i, a->map.info_len, a->infooff, strerror(errno));
>>  			return -errno;
>>  		}
>>  
>>  		a->map.data_len = a->mapoff - a->dataoff;
>> -		a->map.data = mmap(NULL, a->map.data_len, mmap_flags,
>> -			MAP_SHARED, bttc->fd, a->dataoff);
>> -		if (a->map.data == MAP_FAILED) {
>> +		a->map.data = btt_mmap(bttc, a->map.data_len, a->dataoff);
>> +		if (!a->map.data) {
>>  			err(bttc, "mmap arena[%d].data [sz = %#lx, off = %#lx] failed: %s\n",
>>  				i, a->map.data_len, a->dataoff, strerror(errno));
>>  			return -errno;
>>  		}
>>  
>>  		a->map.map_len = a->logoff - a->mapoff;
>> -		a->map.map = mmap(NULL, a->map.map_len, mmap_flags,
>> -			MAP_SHARED, bttc->fd, a->mapoff);
>> -		if (a->map.map == MAP_FAILED) {
>> +		a->map.map = btt_mmap(bttc, a->map.map_len, a->mapoff);
>> +		if (!a->map.map) {
>>  			err(bttc, "mmap arena[%d].map [sz = %#lx, off = %#lx] failed: %s\n",
>>  				i, a->map.map_len, a->mapoff, strerror(errno));
>>  			return -errno;
>>  		}
>>  
>>  		a->map.log_len = a->info2off - a->logoff;
>> -		a->map.log = mmap(NULL, a->map.log_len, mmap_flags,
>> -			MAP_SHARED, bttc->fd, a->logoff);
>> -		if (a->map.log == MAP_FAILED) {
>> +		a->map.log = btt_mmap(bttc, a->map.log_len, a->logoff);
>> +		if (!a->map.log) {
>>  			err(bttc, "mmap arena[%d].log [sz = %#lx, off = %#lx] failed: %s\n",
>>  				i, a->map.log_len, a->logoff, strerror(errno));
>>  			return -errno;
>>  		}
>>  
>>  		a->map.info2_len = BTT_INFO_SIZE;
>> -		a->map.info2 = mmap(NULL, a->map.info2_len, mmap_flags,
>> -			MAP_SHARED, bttc->fd, a->info2off);
>> -		if (a->map.info2 == MAP_FAILED) {
>> +		a->map.info2 = btt_mmap(bttc, a->map.info2_len, a->info2off);
>> +		if (!a->map.info2) {
>>  			err(bttc, "mmap arena[%d].info2 [sz = %#lx, off = %#lx] failed: %s\n",
>>  				i, a->map.info2_len, a->info2off, strerror(errno));
>>  			return -errno;
>> @@ -977,15 +1018,15 @@ static void btt_remove_mappings(struct btt_chk *bttc)
>>  	for (i = 0; i < bttc->num_arenas; i++) {
>>  		a = &bttc->arena[i];
>>  		if (a->map.info)
>> -			munmap(a->map.info, a->map.info_len);
>> +			btt_unmap(bttc, a->map.info, a->map.info_len);
>>  		if (a->map.data)
>> -			munmap(a->map.data, a->map.data_len);
>> +			btt_unmap(bttc, a->map.data, a->map.data_len);
>>  		if (a->map.map)
>> -			munmap(a->map.map, a->map.map_len);
>> +			btt_unmap(bttc, a->map.map, a->map.map_len);
>>  		if (a->map.log)
>> -			munmap(a->map.log, a->map.log_len);
>> +			btt_unmap(bttc, a->map.log, a->map.log_len);
>>  		if (a->map.info2)
>> -			munmap(a->map.info2, a->map.info2_len);
>> +			btt_unmap(bttc, a->map.info2, a->map.info2_len);
>>  	}
>>  }
>

-- 
Vaibhav Jain <vaibhav@linux.ibm.com>
Linux Technology Center, IBM India Pvt. Ltd.

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