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On Thu, Oct 07, 2004 at 04:31:35AM +0100, Stephen C. Tweedie wrote: > Hi, > > On Wed, 2004-10-06 at 09:01 -0300, Marcelo Tosatti wrote: > > > o If O_NONBLOCK is set, read( ) shall return -1 and set errno to [EAGAIN]. > > > > This implies read(O_NONBLOCK) should never block. > > The spec is usually pretty careful never to come straight out and > require that in all cases, even for true AIO. > > > Maybe your code should pass down __GFP_FAIL in the gfp_mask > > to the page_cache_alloc() to avoid blocking reclaiming pages, > > and possibly pass info down to the block layer > > "if this is going to block, fail". > > It's not just the page allocation that can block, though. Readahead > requires us to map the buffers being read before we submit the async > read, so we can still block reading indirect blocks. If we want to > avoid submitting that extra synchronous IO, then either O_NONBLOCK needs > to avoid readahead entirely for non-present pages, or the readahead > itself needs to know that it's a O_NONBLOCK IO and fail cleanly if the > metadata is not in cache. Hi Stephen! Oh yes, theres also the indirect blocks which we might need to read from disk. Now the question is, how strict should the O_NONBLOCK implementation be in reference to "not blocking" ? Maybe Jeff's currently implementation is just fine avoiding the potential block at !PageUptodate. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ | ||||||||||
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