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Support page sizes larger than 4K #430

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7759e4d
Support page sizes larger than 4K in snapshot COW tracking
dzianisbely Jun 21, 2026
3b83141
Fix COW data corruption on kernels with PAGE_SIZE > 4K
dzianisbely Jun 21, 2026
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16 changes: 12 additions & 4 deletions src/bio_helper.c
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Original file line number Diff line number Diff line change
Expand Up @@ -430,7 +430,11 @@ static void __on_bio_read_complete(struct bio *bio, int err)
#ifndef HAVE_BVEC_ITER
//#if LINUX_VERSION_CODE < KERNEL_VERSION(3,14,0)
for (i = 0; i < bio->bi_vcnt; i++) {
bio->bi_io_vec[i].bv_len = PAGE_SIZE;
if (map && (i + 1) * PAGE_SIZE > map->size) {
bio->bi_io_vec[i].bv_len = map->size - i * PAGE_SIZE;
} else {
bio->bi_io_vec[i].bv_len = PAGE_SIZE;
}
bio->bi_io_vec[i].bv_offset = 0;
}
#endif
Expand Down Expand Up @@ -695,13 +699,14 @@ void bio_free_clone(struct bio *bio)
*/
int bio_make_read_clone(struct bio_set *bs, struct tracing_params *tp,
struct bio *orig_bio, sector_t sect, unsigned int pages,
struct bio **bio_out, unsigned int *bytes_added)
unsigned int tail, struct bio **bio_out, unsigned int *bytes_added)
{
int ret;
struct bio *new_bio;
struct page *pg;
unsigned int i;
unsigned int bytes;
unsigned int actual_size = PAGE_SIZE;
unsigned int total = 0;
#ifdef BIO_MAX_PAGES
unsigned int actual_pages =
Expand Down Expand Up @@ -758,9 +763,12 @@ int bio_make_read_clone(struct bio_set *bs, struct tracing_params *tp,
goto error;
}

if (i == pages - 1 && tail != 0)
actual_size = PAGE_SIZE - tail;

// add the page to the bio
bytes = bio_add_page(new_bio, pg, PAGE_SIZE, 0);
if (bytes != PAGE_SIZE) {
bytes = bio_add_page(new_bio, pg, actual_size, 0);
if (bytes != actual_size) {
__free_page(pg);
break;
}
Expand Down
2 changes: 1 addition & 1 deletion src/bio_helper.h
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Original file line number Diff line number Diff line change
Expand Up @@ -149,7 +149,7 @@ void bio_free_clone(struct bio *bio);

int bio_make_read_clone(struct bio_set *bs, struct tracing_params *tp,
struct bio *orig_bio, sector_t sect, unsigned int pages,
struct bio **bio_out, unsigned int *bytes_added);
unsigned int tail, struct bio **bio_out, unsigned int *bytes_added);

#ifdef HAVE_BIO_ENDIO_INT
void dattobd_bio_endio(struct bio *bio, int err);
Expand Down
2 changes: 1 addition & 1 deletion src/cow_manager.h
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Expand Up @@ -14,7 +14,7 @@
#include <stdint.h>
#endif

#define COW_SECTION_SIZE 4096
#define COW_SECTION_SIZE PAGE_SIZE

#define cow_write_filler_mapping(cm, pos) __cow_write_mapping(cm, pos, 1)
extern const unsigned long dattobd_cow_ext_buf_size;
Expand Down
7 changes: 4 additions & 3 deletions src/snap_handle.c
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Expand Up @@ -240,6 +240,7 @@ int snap_handle_write_bio(const struct snap_device *dev, struct bio *bio)
{
int ret;
char *data;
char *block_data;
sector_t start_block, end_block = SECTOR_TO_BLOCK(bio_sector(bio));
struct bio_vec *bvec;
#ifdef HAVE_BVEC_ITER_ALL
Expand All @@ -266,10 +267,10 @@ int snap_handle_write_bio(const struct snap_device *dev, struct bio *bio)
// map the page into kernel space
data = kmap(bvec->bv_page);

// loop through the blocks in the page
for (; start_block < end_block; start_block++) {
block_data = data + bvec->bv_offset;
for (; start_block < end_block; start_block++, block_data += COW_BLOCK_SIZE) {
// pass the block to the cow manager to be handled
ret = cow_write_current(dev->sd_cow, start_block, data);
ret = cow_write_current(dev->sd_cow, start_block, block_data);
if (ret) {
LOG_ERROR(ret,"memory demands %llu, memory saved before crash %llu",number_of_blocks*COW_BLOCK_SIZE,saved_blocks*COW_BLOCK_SIZE);
kunmap(bvec->bv_page);
Expand Down
10 changes: 6 additions & 4 deletions src/tracer.c
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Original file line number Diff line number Diff line change
Expand Up @@ -122,7 +122,7 @@ static int snap_trace_bio(struct snap_device *dev, struct bio *bio)
struct bio *new_bio = NULL;
struct tracing_params *tp = NULL;
sector_t start_sect, end_sect;
unsigned int bytes, pages;
unsigned int bytes, pages, tail;

// if we don't need to cow this bio just call the real mrf normally
if (!bio_needs_cow(bio, dev->sd_cow_inode) || tracer_read_fail_state(dev))
Expand All @@ -144,7 +144,9 @@ static int snap_trace_bio(struct snap_device *dev, struct bio *bio)
dev->sd_sect_off,
SECTORS_PER_BLOCK) +
dev->sd_sect_off;
pages = (end_sect - start_sect) / SECTORS_PER_PAGE;

pages = ROUND_UP(end_sect - start_sect, SECTORS_PER_PAGE) / SECTORS_PER_PAGE;
tail = pages * PAGE_SIZE - (end_sect - start_sect) * SECTOR_SIZE;

// allocate tracing_params struct to hold all pointers we will need
// across contexts
Expand All @@ -164,7 +166,7 @@ static int snap_trace_bio(struct snap_device *dev, struct bio *bio)
// allocate and populate read bio clone. This bio may not have all the
// pages we need due to queue restrictions
ret = bio_make_read_clone(dev_bioset(dev), tp, bio, start_sect, pages,
&new_bio, &bytes);
tail, &new_bio, &bytes);
if (ret)
goto error;

Expand All @@ -188,7 +190,7 @@ static int snap_trace_bio(struct snap_device *dev, struct bio *bio)

// if our bio didn't cover the entire clone we must keep creating bios
// until we have
if (bytes / PAGE_SIZE < pages) {
if (bytes < (end_sect - start_sect) * SECTOR_SIZE) {
start_sect += bytes / SECTOR_SIZE;
pages -= bytes / PAGE_SIZE;
continue;
Expand Down