Martin Maletinsky wrote: > > 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? Yes. > (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. You really mean "process" here, because threads share VM (so no copy-on-write, etc.) > (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). If Process B tries to write VP_1, it will get a page fault (either due to the page not yet being mapped, or due to the copy-on-write state), find the page in the page cache, and get it its own copy mapped into its VM. Unless we -know- a page is shareable, the writing process always gets its own copy, and any mappings to the old page are left unchanged. > (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). Nope. B gets its own copy, A uses the existing one. So A's read() completes normally. Cheers, -- Joe # "You know how many remote castles there are along the # gorges? You can't MOVE for remote castles!" - Lu Tze re. Uberwald # Linux MM docs: http://home.earthlink.net/~jknapka/linux-mm/vmoutline.html - 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/
