Asynchronous Block I/O request

In the Linux kernel, Block I/O request are asynchronous. It means that when you call submit_bio(READ/WRITE, bio); or generic_make_request(…), the function will (most probably) return directly, and of course, the read is not done. So after calling bio_submit(READ…); you absolutely cannot read the content of a page added by bio_add_page().

So, how to know when it is finished? You have to use bio->bi_end_io pointer function. You have to set this pointer to a function which will be called when the read has been done.

void myReadIsFinished(struct bio* bio, int error) {
//Read is finished, do something with the bio content


bio->bi_end_io = &myReadIsFinished;
bio_submit(READ, bio);

bio->bi_private allows you to store some pointer with the bio. Use it to know what you tried to read.

Limiting the incoming Block I/O requests to a device driver/md device

When implementing a device driver or a MD device which can receive Block I/O (struct bio in the kernel), you can receive BIO of nearly any size, with any number of segments (segments are discontinued parts of a common buffer, defined in a bio request). You may want to limit :

– The number of segments you can receive with

blk_queue_max_segments(queue, X);

Where X is the number of segments per struct bio

– The maximal size of the request :

blk_queue_max_hw_sectors(queue, Y);

Where Y is the maximal size in sectors

For a md device, the queue can be recovered with mddev->queue

The combination of the two allows to limit ensure that all bio request have always maximum X segments for a maximal size of Y sectors.

It is used in raid0 with Y=mddev->chunk_sectors to ensure that no request is bigger than one chunk, so any request cross at most one chunk boundary. And with X=1, it allows to use the bio_split function to split a request which would span on the two sides of a chunk boundary.

Automatically find all returned e-mails from “Undelivered messages”

If you’ve got your mails under a text format in a folder (like the unix Maildir) you can use this command to extract the e-mails with a 550 return error.


cat * | grep --text -Pzoi '([a-z0-9._-]+@[a-z0-9.]+)(?=.*host.*said.*55[0-9])'


The first command in the pipe, cat, send all files content to the next command in the pipe : grep. Grep is removing everything except what is an adress, and only if it is followed by “host * said * 55[0-9] ” where * can be everything and [0-9] is a number between 0 and 9. We also use the –text parameter because some mails could contain binary data.


As grep give you the mail separated by new line, and an sql command takes a list of strings separated by comma, you can copy the list in gedit or notepad++ and use search->replace to change them in the format ‘mail1’, ‘mail2’, … You have to put “(.*)” in the search field, “‘\1’,” in the replace by field, and select “regular expression”. You then place the result in the parenthesis after IN, in SQL command below :


UPDATE contact SET mail='' WHERE mail IN ('', '')