Hi, I more or less understand the Linux memory management system, but I have a very limited understanding of how I/O works. I am interested in understanding, how I/O is synchronized with the memory managment, in particular I have the following questions: (1) When a process wants to read data from an I/O device (e.g. disk) into a buffer in it's address space, (I assume) the pages containing that buffer must be 'locked', i.e. prevented from being paged out, while the data transfer is going on. As far as I understood from quickly browsing through the 'Managing I/O Devices' chapter in 'Understanding the Linux Kernel', this is achieved by setting the PG_locked flag in the corresponding page frame descriptor. Is this correct? (2) How is I/O synchronized with the copy-on-write mechanism? Imagine the following scenario: (i) Two threads A and B share their virtual address space (i.e. the CLONE_VM flag was set when A called clone() to create B). (ii) Thread A starts reading data from a hard disk into a buffer BUF which lies within virtual page VP_1 (which at that moment corresponds to the physical page PP_1). In order to service the read request, the physical page PP_1 is locked (?) and DMA is set up, to transfer data from the harddisk into the correct location within physical page PP_1. (iii) Thread A sleeps until termination of it's read request. (iv) Meanwhile thread B calls fork(), which results in write protection of the virtual page VP_1 (in order to enforce a copy on write at a subsequent write access). (v) Thread B writes to the virtual page VP_1 (outside the buffer BUF). This results in a page fault, and subsequently in a copy on write (COW) of the virtual page VP_1. As a consequence of the COW, the content of PP_1 is copied into a new physical page PP_2 and virtual page VP_1 is associated to PP_2 (by modifying the respective page table entry for thread A and B). Note, that the data transfer from disk is still going on and the data is written into the physical page PP_1. (vi) The data transfer from disk terminates and thread A wakes up. Thread A would now expect to find the disk data in buffer BUF. However, when it reads the buffer BUF within virtual page VP_1, it will access the physical page PP_2. Therefore it will not 'see' the data in the buffer BUF that has been transferred after the COW took place in (v). I guess that I made some wrong assumptions in my scenario, since I can hardly imagine, that it is acceptable that thread A's read request is incorrectly handled, because thread B calls fork(). Please help me understand how the kernel manages to correctly process both calls. regards Martin -- Supercomputing System AG email: maletinsky@scs.ch Martin Maletinsky phone: +41 (0)1 445 16 05 Technoparkstrasse 1 fax: +41 (0)1 445 16 10 CH-8005 Zurich - Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ IRC Channel: irc.openprojects.net / #kernelnewbies Web Page: http://www.kernelnewbies.org/
