I am newbie to Linux kernel and just started to know how zram works. Initial testing, I am seeing that READ is issued before WRITE just after the zram is being initialized. But I am just eager to know, why this is so ?
As an activity I took the dump_stack() and followed the path form where to how this zram read is being performed.
zram get to know this info whether it has to do READ or WRITE operation on issued bio->bi_rw. Code flow is like that zram_make_request API is being called from create_device in zram driver. And zram_make_request internally called __zram_make_request which called the zram_bvec_rw API.
In zram_bvec_rw API check the available info of bio->bi_rw and correspondingly issued the READ and WRITE call.
Now, in this case what is happening: READ is being encapsulated inside bio struct itself. As triage I found that submit_bh fills all the entry of bio and issued the submit_bio.
I was wondering who is actually sets the bio->bi_rw as READ. By enabling the few prints I found that ll_rw_block API is being called by __block_write_begin with READ, later ll_rw_block calls the submit_bh API where rest of bio struct entries are filled.
But I am still not getting the answer why READ is issued for ll_rw_block from __block_write_begin ?
- zram driver:
if (!buffer_uptodate(bh) && !buffer_delay(bh) &&
!buffer_unwritten(bh) &&
(block_start < from || block_end > to)) {
ll_rw_block(READ, 1, &bh);
*wait_bh++=bh;
}
buffer_uptodate(bh), /* Contains valid data */
buffer_delay(bh), /* Buffer is not yet allocated on disk */
buffer_unwritten(bh), /* Buffer is allocated on disk but not written */
Please can someone give an explaination/answer to my question ?
How I am concluding that read is perfomed before write ??
I just check the num_reads and num_writes count. And num_reads count is set to 1 while num_writes is found 0 when we do mkswap /dev/block/zram0 and after calling the swapon /dev/block/zram0 the final counts are num_reads = 2 and num_writes=1.
NOTE: This is the case when we don't performing any additional zram activity. We got this behavior in case as explained above.
