Hi Sergey,
On Tue, Apr 04, 2017 at 11:17:06AM +0900, Sergey Senozhatsky wrote:
> Hello,
>
> On (04/03/17 14:17), Minchan Kim wrote:
> > +static bool zram_special_page_read(struct zram *zram, u32 index,
> > + struct page *page,
> > + unsigned int offset, unsigned int len)
> > +{
> > + struct zram_meta *meta = zram->meta;
> > +
> > + bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> > + if (unlikely(!meta->table[index].handle) ||
> > + zram_test_flag(meta, index, ZRAM_SAME)) {
> > + void *mem;
> > +
> > + bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> > + mem = kmap_atomic(page);
> > + zram_fill_page(mem + offset, len, meta->table[index].element);
> > + kunmap_atomic(mem);
> > + return true;
> > + }
> > + bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> > +
> > + return false;
> > +}
> > +
> > +static bool zram_special_page_write(struct zram *zram, u32 index,
> > + struct page *page)
> > +{
> > + unsigned long element;
> > + void *mem = kmap_atomic(page);
> > +
> > + if (page_same_filled(mem, &element)) {
> > + struct zram_meta *meta = zram->meta;
> > +
> > + kunmap_atomic(mem);
> > + /* Free memory associated with this sector now. */
> > + bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> > + zram_free_page(zram, index);
> > + zram_set_flag(meta, index, ZRAM_SAME);
> > + zram_set_element(meta, index, element);
> > + bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> > +
> > + atomic64_inc(&zram->stats.same_pages);
> > + return true;
> > + }
> > + kunmap_atomic(mem);
> > +
> > + return false;
> > +}
>
> zram_special_page_read() and zram_special_page_write() have a slightly
> different locking semantics.
>
> zram_special_page_read() copy-out ZRAM_SAME page having slot unlocked
> (can the slot got overwritten in the meantime?), while
IMHO, yes, it can be overwritten but it doesn't make corruption of kernel.
I mean if such race happens, it's user fault who should protect the race.
zRAM is dumb block device so it can read/write block user request but
one thing we should keep the promise is it shouldn't corrupt the kernel.
Such pov, zram_special_page_read wouldn't be a problem to return
stale data, I think.
> zram_special_page_write() keeps the slot locked through out the entire
> operation.
zram_special_page_write is something different because it updates
zram_table's slot via zram_set_[flag|element] so it should be protected
by zram.
>
> > static void zram_meta_free(struct zram_meta *meta, u64 disksize)
> > {
> > size_t num_pages = disksize >> PAGE_SHIFT;
> > @@ -504,169 +548,104 @@ static void zram_free_page(struct zram *zram,
> > size_t index)
> > zram_set_obj_size(meta, index, 0);
> > }
> >
> > -static int zram_decompress_page(struct zram *zram, char *mem, u32 index)
> > +static int zram_decompress_page(struct zram *zram, struct page *page, u32
> > index)
> > {
> > - int ret = 0;
> > - unsigned char *cmem;
> > - struct zram_meta *meta = zram->meta;
> > + int ret;
> > unsigned long handle;
> > unsigned int size;
> > + void *src, *dst;
> > + struct zram_meta *meta = zram->meta;
> > +
> > + if (zram_special_page_read(zram, index, page, 0, PAGE_SIZE))
> > + return 0;
> >
> > bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> > handle = meta->table[index].handle;
> > size = zram_get_obj_size(meta, index);
> >
> > - if (!handle || zram_test_flag(meta, index, ZRAM_SAME)) {
> > - bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> > - zram_fill_page(mem, PAGE_SIZE, meta->table[index].element);
> > - return 0;
> > - }
> > -
> > - cmem = zs_map_object(meta->mem_pool, handle, ZS_MM_RO);
> > + src = zs_map_object(meta->mem_pool, handle, ZS_MM_RO);
> > if (size == PAGE_SIZE) {
> > - copy_page(mem, cmem);
> > + dst = kmap_atomic(page);
> > + copy_page(dst, src);
> > + kunmap_atomic(dst);
> > + ret = 0;
> > } else {
> > struct zcomp_strm *zstrm = zcomp_stream_get(zram->comp);
> >
> > - ret = zcomp_decompress(zstrm, cmem, size, mem);
> > + dst = kmap_atomic(page);
> > + ret = zcomp_decompress(zstrm, src, size, dst);
> > + kunmap_atomic(dst);
> > zcomp_stream_put(zram->comp);
> > }
> > zs_unmap_object(meta->mem_pool, handle);
> > bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> >
> > /* Should NEVER happen. Return bio error if it does. */
> > - if (unlikely(ret)) {
> > + if (unlikely(ret))
> > pr_err("Decompression failed! err=%d, page=%u\n", ret, index);
> > - return ret;
> > - }
> >
> > - return 0;
> > + return ret;
> > }
> >
> > static int zram_bvec_read(struct zram *zram, struct bio_vec *bvec,
> > - u32 index, int offset)
> > + u32 index, int offset)
> > {
> > int ret;
> > struct page *page;
> > - unsigned char *user_mem, *uncmem = NULL;
> > - struct zram_meta *meta = zram->meta;
> > - page = bvec->bv_page;
> >
> > - bit_spin_lock(ZRAM_ACCESS, &meta->table[index].value);
> > - if (unlikely(!meta->table[index].handle) ||
> > - zram_test_flag(meta, index, ZRAM_SAME)) {
> > - bit_spin_unlock(ZRAM_ACCESS, &meta->table[index].value);
> > - handle_same_page(bvec, meta->table[index].element);
> > + page = bvec->bv_page;
> > + if (zram_special_page_read(zram, index, page, bvec->bv_offset,
> > + bvec->bv_len))
>
> so, I think zram_bvec_read() path calls zram_special_page_read() twice:
>
> a) direct zram_special_page_read() call
>
> b) zram_decompress_page()->zram_special_page_read()
>
> is it supposed to be so?
Yes, Because zram_decompress_page is called by zram_bvec_write
in case of partial IO. Maybe, we makes it simple with removing
zram_special_page_read in zram_bvec_read. I will look.
