[PATCH v6 1/4] cpumask: introduce cpumap_print_to_buf to support large bitmask and list

[Barry Song]

From: Tian Tao <tiantao6@xxxxxxxxxxxxx>

The existing cpumap_print_to_pagebuf() is used by cpu topology and other
drivers to export hexadecimal bitmask and decimal list to userspace by
sysfs ABI.

Right now, those drivers are using a normal attribute for this kind of
ABIs. A normal attribute typically has show entry as below:

static ssize_t example_dev_show(struct device *dev,
                struct device_attribute *attr, char *buf)
{
	...
        return cpumap_print_to_pagebuf(true, buf, &pmu_mmdc->cpu);
}
show entry of attribute has no offset and count parameters and this
means the file is limited to one page only.

cpumap_print_to_pagebuf() API works terribly well for this kind of
normal attribute with buf parameter and without offset, count:

static inline ssize_t
cpumap_print_to_pagebuf(bool list, char *buf, const struct cpumask *mask)
{
        return bitmap_print_to_pagebuf(list, buf, cpumask_bits(mask),
                                      nr_cpu_ids);
}

The problem is once we have many cpus, we have a chance to make bitmask
or list more than one page. Especially for list, it could be as complex
as 0,3,5,7,9,...... We have no simple way to know it exact size.

It turns out bin_attribute is a way to break this limit. bin_attribute
has show entry as below:
static ssize_t
example_bin_attribute_show(struct file *filp, struct kobject *kobj,
             struct bin_attribute *attr, char *buf,
             loff_t offset, size_t count)
{
        ...
}

With the new offset and count parameters, this makes sysfs ABI be able
to support file size more than one page. For example, offset could be
>= 4096.

This patch introduces cpumap_print_to_buf() so that those drivers can
move to bin_attribute to support large bitmask and list. In result,
we have to pass the corresponding parameters from bin_attribute to this
new API.

Signed-off-by: Tian Tao <tiantao6@xxxxxxxxxxxxx>
Reviewed-by: Jonathan Cameron <jonathan.cameron@xxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxxxxxxxx>
Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: Stefano Brivio <sbrivio@xxxxxxxxxx>
Cc: Alexander Gordeev <agordeev@xxxxxxxxxxxxx>
Cc: "Ma, Jianpeng" <jianpeng.ma@xxxxxxxxx>
Cc: Yury Norov <yury.norov@xxxxxxxxx>
Cc: Valentin Schneider <valentin.schneider@xxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
---
 -v6:
  -minor cleanup doc according to Andy Shevchenko's comment;
  -take bitmap_print_to_buf back according to Yury Norov's comment and
   fix the documents;
  -Sorry, Yury, I don't think it is doable to move memory allocation
   to drivers.
   Considering a driver like topology.c, we have M CPUs and each CPU
   has N various nodes like core_siblings, package_cpus, die_cpus etc,
   we can't know the size of each node of each CPU in advance. The best
   time to get the size of each node is really when users read the sysfs
   node. otherwise, we have to scan M*N nodes in drivers in advance to
   figure out the exact size of buffers we need.
   On the other hand, it is crazily tricky to ask a bundle of drivers to
   find a proper place to save the pointer of allocated buffers so that
   they can be re-used in second read of the same bin_attribute node.
   And I doubt it is really useful to save the address of buffers
   somewhere. Immediately freeing it seems to be a correct option to
   avoid runtime waste of memory. We can't predict when users will read
   topology ABI and which node users will read.
   Finally, reading topology things wouldn't be the really cpu-bound
   things in user applications, hardly this kind of ABI things can be
   a performance bottleneck. Users use numactl and lstopo commands to
   read ABIs but nobody will do it again and again. And a normal
   application won't read topology repeatly. So the overhead caused by
   malloc/free in the new bitmap API doesn't really matter.
   if we really want a place to re-used the buffer and avoid malloc/free,
   it seems this should be done in some common place rather than each
   driver. still it is hard to find the best place.

   Thanks for the comments of Yury and Andy in v5.

 include/linux/bitmap.h  |  2 ++
 include/linux/cpumask.h | 24 +++++++++++++++++++++++
 lib/bitmap.c            | 43 +++++++++++++++++++++++++++++++++++++++++
 3 files changed, 69 insertions(+)

diff --git a/include/linux/bitmap.h b/include/linux/bitmap.h
index a36cfcec4e77..0de6effa2797 100644
--- a/include/linux/bitmap.h
+++ b/include/linux/bitmap.h
@@ -226,6 +226,8 @@ void bitmap_copy_le(unsigned long *dst, const unsigned long *src, unsigned int n
 unsigned int bitmap_ord_to_pos(const unsigned long *bitmap, unsigned int ord, unsigned int nbits);
 int bitmap_print_to_pagebuf(bool list, char *buf,
 				   const unsigned long *maskp, int nmaskbits);
+int bitmap_print_to_buf(bool list, char *buf, const unsigned long *maskp,
+			int nmaskbits, loff_t off, size_t count);
 
 #define BITMAP_FIRST_WORD_MASK(start) (~0UL << ((start) & (BITS_PER_LONG - 1)))
 #define BITMAP_LAST_WORD_MASK(nbits) (~0UL >> (-(nbits) & (BITS_PER_LONG - 1)))
diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index bfc4690de4f4..8a89d133fa2d 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -983,6 +983,30 @@ cpumap_print_to_pagebuf(bool list, char *buf, const struct cpumask *mask)
 				      nr_cpu_ids);
 }
 
+/**
+ * cpumap_print_to_buf  - copies the cpumask into the buffer
+ * @list: indicates whether the cpumap must be list
+ *      true:  print in decimal list format
+ *      false: print in hexadecimal bitmask format
+ * @mask: the cpumask to copy
+ * @buf: the buffer to copy into
+ * @off: in the string from which we are copying, We copy to @buf
+ * @count: the maximum number of bytes to print
+ *
+ * The function copies the cpumask into the buffer either as comma-separated
+ * list of cpus or hex values of cpumask; Typically used by bin_attribute to
+ * export cpumask bitmask and list ABI.
+ *
+ * Returns the length of how many bytes have been copied.
+ */
+static inline ssize_t
+cpumap_print_to_buf(bool list, char *buf, const struct cpumask *mask,
+		loff_t off, size_t count)
+{
+	return bitmap_print_to_buf(list, buf, cpumask_bits(mask),
+			nr_cpu_ids, off, count);
+}
+
 #if NR_CPUS <= BITS_PER_LONG
 #define CPU_MASK_ALL							\
 (cpumask_t) { {								\
diff --git a/lib/bitmap.c b/lib/bitmap.c
index 9401d39e4722..c64baa3a8606 100644
--- a/lib/bitmap.c
+++ b/lib/bitmap.c
@@ -487,6 +487,49 @@ int bitmap_print_to_pagebuf(bool list, char *buf, const unsigned long *maskp,
 }
 EXPORT_SYMBOL(bitmap_print_to_pagebuf);
 
+/**
+ * bitmap_print_to_buf  - convert bitmap to list or hex format ASCII string
+ * @list: indicates whether the bitmap must be list
+ *      true:  print in decimal list format
+ *      false: print in hexadecimal bitmask format
+ * @buf: buffer into which string is placed
+ * @maskp: pointer to bitmap to convert
+ * @nmaskbits: size of bitmap, in bits
+ * @off: in the string from which we are copying, We copy to @buf
+ * @count: the maximum number of bytes to print
+ *
+ * The role of cpumap_print_to_buf() and cpumap_print_to_pagebuf() is similar,
+ * the difference is that bitmap_print_to_pagebuf() mainly serves sysfs
+ * attribute with the assumption the destination buffer is exactly one page
+ * aligned with PAGE_SIZE and it won't be more than one page, thus,
+ * bitmap_print_to_pagebuf() needs neither offset to copy from nor count
+ * which is the length we are going to copy. cpumap_print_to_buf(), on the
+ * other hand, mainly serves bin_attribute which doesn't work with exact
+ * one page, and it has explicit parameters like "offset" to copy from and
+ * "count" bytes to copy. So cpumap_print_to_buf() can break the size limit
+ * of converted decimal list and hexadecimal bitmask. And buf doesn't have
+ * to be exactly one page.
+ *
+ * Returns the number of characters actually printed to @buf
+ */
+int bitmap_print_to_buf(bool list, char *buf, const unsigned long *maskp,
+		int nmaskbits, loff_t off, size_t count)
+{
+	const char *fmt = list ? "%*pbl\n" : "%*pb\n";
+	ssize_t size;
+	void *data;
+
+	data = kasprintf(GFP_KERNEL, fmt, nmaskbits, maskp);
+	if (!data)
+		return -ENOMEM;
+
+	size = memory_read_from_buffer(buf, count, &off, data, strlen(data) + 1);
+	kfree(data);
+
+	return size;
+}
+EXPORT_SYMBOL(bitmap_print_to_buf);
+
 /*
  * Region 9-38:4/10 describes the following bitmap structure:
  * 0	   9  12    18			38	     N
-- 
2.25.1